C.G.
I'm not sure how closely related this story is to your situation, but I thought I'd share it because it is positive. I used the work in a child development center in an infant/toddler room and there was a little boy who started when he was about 3 or 4 months old. When you picked him up, he felt like a rag doll. His head bent back if not supported and he was totally limp when picked up. He also drooled all the time because his mouth hung open and his face sagged a little. His tummy bulged out from his muscles not being tight and his chest caved in. All the teachers kind of wondered about his situation and eventually mentioned concerns to his mother. She asked the doctor about this and he said he was "double jointed." I was so mad that this doctor said this. I have no medical degree, just a masters in early childhood, but even I could tell that this baby had low muscle tone.
The good news is that this baby was very active and was always moving and trying to get around. With the way his muscles were, it seemed impossible that he would ever be able to sit or stand or crawl on his own. However, he reached all of his milestones either early or on time! He was strong when he used his muscles, just limp when he wasn't using them. I'm not sure if he stayed on track because he was so active, and I'm not sure how a laid-back baby would be with this condition, but he did great!
So, I'm sure the therapists will know exactly how to help your baby. And, if you don't feel comfortable, it's always ok to get a second opinion if you don't feel like they are helping as much as they can.
You might want to give him plenty of floor time so he can practice using his muscles. Do some exercises with him that help with his strength--lay him flat and gently sit him up by lifting his arms so that he has to hold his own head up while sitting up. I'm sure the therapists will give you lots of things you can do at home too.
I'm sure everything will turn out fine. Best wishes!