advice needed for a first time mom with a 5 week premie babe

my L.O was born 3 weeks early but was 2-3 weeks behind the growth chart while in womb. he is now 3 months but the only milestone he's even close to hitting is rolling over he gets on his side and is almost there we do tummy time but his head his so big compared to his body I think that's a big factor. he does so well on the boppy pillow and if he's laying on your chest but if he's on the floor he does nothing. I'm so scared I'm not doing enough to help his development. I'm just not sure what else I can do if I can even do anything.