6 month old milestones

So my baby was born 7/4/20. So far all he can do is suck his toes and sit for like 30 seconds before he tips over. Im a little concerned, he doesnt even try to roll over. My other 2 were doing a bit more by now when They were his age. His dr is also worried that he doesnt like to stand, he literally will scrunch his legs up as soon as he feels his feet touch a surface. If he does manage to keep his legs done he wont put weight on 1 leg. Ive been trying to get him to learn how by holding him up on my legs or on the exersaucer but i dont see much improvement. Dr said if nothing changed by his 6 month Check up he wanted to send him to a specialist. Has anyone else had this issue?