7 y/o ADHD/Delayed boy peeing everywhere :/

Just to set aside some confusion, my name is Jozef (Yozif), I am 22, and I am new to the forum. I am a member of a big family... Lol... Lots of adopted boys, myself included. I have Asperger's (or whatever you kids are calling it these days) and I live at home. My mother has been away for a week on a mandatory stay visiting another little boy who is having difficulty fitting into a family (she wants to bring him home). I have been keeping an eye on my little brother whom I'll call Vahni. This arrangement isn't unusual... My mom will often go away for a weekend for work or volunteer stuff or whatever, and I will stay here with my brother. He is usually fine. He often cries when she leaves, and he is often very clingy and very needy while she is gone, but he isn't ever bad or anything. He asks to sleep with me sometimes because he is feeling lonely or w/e, but he never acts out more than usual. This time, mom has been gone for a week, and he seems completely distraught. He knew well ahead of time how long she would be gone, and I tried hard to keep him occupied and feeling loved despite her absence. One day I walked down the hallway and sort of stopped and did a double take and realized there was a big wet spot on the carpet in my mom's room. I went in to investigate and it was definitely pee. We don't have a dog, so I knew where it came from. Vahni isn't always 100% sure of whether or not he can hold it and I just cleaned it up and assumed it was an accident, didn't say anything to him about it so as not to embarrass him. Later that night, when I went to get him ready for bed, he was sitting on his beanbag with "that look" on his face, like he very obviously did something wrong and isn't sure whether I figured it out yet or not. I asked him "Do you wanna talk about anything?" and he started crying and said he was sorry. I still had no idea what he did, so I asked "And what are you sorry for?" and he said "I'm sorry I peed in the closet." I acted like I knew all along, and asked him why he did it, and he just kept saying he didn't know why he did it, but he was sorry, and he wouldn't do it anymore. So I made him use the bathroom and just put him to bed like any other night. Sure enough, he had peed in the corner of my mom's closet, but didn't get anything wet aside from the wall and carpet. I cleaned it up and went to bed. Early early morning, kiddo woke me up and said he had wet the bed, which is not unusual for him, but when I turned the light on I noticed his PJ's were still on and they were totally dry. Went into his room and his bed was wet, but it was like he had stood on it and peed at the foot of the bed, on the top layer. At the time it was obvious that that was what had happened, but I was so **** tired I just pretended like I didn't know. Changed his bedding out for him and went back to sleep. Next day nothing happened and I figured he had just had a bad day, was stressed out, etc. We went for a bike ride and he seemed happy. Put him to bed as usual, and maybe an hour after that, he woke me up and said he had wet the bed, which I was immediately suspicious about because it had been so soon after bed time and he is so good about using the bathroom before bed. Got in there and holy crap, the whole bed was like a sprinkler had gone off, like he was just spraying it around. I made him sit on his beanbag and I told him, very plainly, that I knew he did it on purpose, and that it was not okay. I told him he needed to go to the bathroom and stay in there until he peed in the toilet, and he got a little teary but did as he was told. I cleaned up the mess, changed out the bedding and told him he could come out. I told him I was disappointed in him, and that mommy would have been very upset. I could hear him crying in there for quite a while after and I was certain beyond all doubt that he was not going to do it again. Yesterday he peed on my mom's bed as soon as he got home from school. I didn't catch it because I was downstairs doing litterboxes but the bedding is snow white and it was easy to see. When I went to find him, he was sitting on his beanbag just like before, and never have I had such an urge to spank him in my life. I lost my **** a little bit and asked him what was wrong with him / why would he do something so awful. He started crying and said he didn't know, which was really not a satisfying answer for me. I made him get up and learn how to wash bedding, which he did not like. He cried the whole time. After everything was clean, he was very quiet and wouldn't look me in the eye. I told him I was sorry for yelling, but that he needed to stop doing this because it is unacceptable. He said he was sorry and he wouldn't do it again. We popped in a movie (Wreck it Ralph of course) and he seemed back to himself after. Put him to bed. He woke me up around midnight saying he had wet the bed. Seemed legit this time so I gave him the benefit of the doubt, hugs, change bedding, back to bed. When I got him up for school this morning, he was really quiet and didn't finish his breakfast which is weird for him. He got on the bus and I decided to walk through mom's room and make sure everything was okay. Nothing amiss. Went into his room and found he had peed in TWO places. Cleaned it up. When he got home I told him I knew he peed in his room, and if it happens again, he is going to lose his gameboy. That made him frantic and he swore up and down it would not happen again. When I went in to put him to bed tonight, I discovered he has been peeing, lord knows how many times, behind his dresser. Kept looking around and he has also peed behind his nightstand and behind his toybox. All of this has just started happening since my mom left and I haven't told her about it because I don't want her to rush home frantically. She is kind of a worry wart. I have a friend who works in therapy and he thinks this is a coping thing and that it will likely stop once my mom comes back - but what I want to know is if adding a new little boy to the family is going to aggravate the problem. Is this an angsty acting out / rebelling thing or is this an anxiety / attention thing? If he misses her, it seems silly that he would pee all over her room. I guess I just don't understand what is going on in his head. It would help me a lot if I could get some insight / outside perspective as to why he may be doing this so that I can help him get over it. If my mom finds out she is going to freak.