X-post -- need help with letter to bio father

We adopted our children through the foster care system. We picked up my youngest son from the hospital when he was a newborn and only "met" his birth parents once at the case plan meeting. My son is now almost 3 and we recently discovered that his biological father is incarcerated. Since neither parent participated in the case plan process beyond the first meeting we have very limited information about the family medical history. I wrote his biological father a letter and asked for medical information. He wrote back what seems to be a very heartfelt letter thanking us for adopting his son and for reaching out to him. He also filled out the medical history questionnaire I sent and was very thorough. He asked that we write back to him and send a picture of my son. My husband and I have always said that we would be open to sending letters and pictures if any of our children's biological parents reached out and seemed genuinely interested. So (finally!) here's my question. I want to let my son's biological father know that we are willing to send photos and letters and also to receive letters from him to give to my son when he is older. I want to also let him know, however, that letters and photos are probably the extent of openness that we would be willing to participate in. He hasn't asked for anything more than that, but I want to be fair and honest so that he doesn't expect more. Should I leave that out of the letter and be prepared to address it in the future or sort of set the ground rules from the beginning? He will be in jail 300 miles away for another 4 years so it's not like he would ask for a meeting anytime soon. I've never done this so I'd love some feedback from someone who has been there, done that. Thanks!