Doctor said my son is behind

My son is 9 months old, he army crawls.

Sits up if put in that position.

Doctor said he is behind but "boys are slower at developing ".

My son was also born at 35w4d .

I feel all kids develop at their own rate but now the doctor has me worried he is delayed because of medical reasons since my oldest is special needs.

What age did your kids do things?

My oldest hit every mile stone at the average time frame but he was also born full term at 40 weeks.

Edit: thanks yall, that makes me feel better.

He rolls EVERYWHERE and army crawls everywhere.

He babbles says dads,mama, ga sound.

He clicks with his tongue, trys to mimic sounds we make , holds his bottle when he wants to. He can rock back and forth in sit position without falling and can catch his self if he starts leaning.

He can pick things up with his fingers,switch items from hand to hand and is VERY observant of everything.

He can even pop out my phone popsocket close it and do it again lol