new to this

I'm a recent bmom, but now I'm starting to feel almost like a foster parent to an older child. Heres the quick version. My bf son -8 yrs old- just started to visit every other weekend. prior to this, no contact with us at all except christmas for about an hour or 2. Well he is ADD and has had basically no disipline in his life. I have an extremely well behaved, calm 6 yr old. My bf's son was starting fires with the sparklers, refused to listen, when he did listen, he refused to complete the task- making the bed (which is even more frustrating because my 6 yr old does it every day without being asked) How do you disipline a boy you dont even really know? How do you get him to follow simple tasks? He is already on meds for it. Especially a boy you dont know who is ADD and also has learning disabilities? His dad refuses to put him in a time out saying He cant sit still for that long. Should I push a time out? Or is there a better way to do this? Positive reinforcement so far hasnt worked either, although we've only had him every other weekend for about 3 months. HELP???