Missing milestones - early intervention?

Starting to get worried my son is getting too far behind in milestones. He’s 11 months, but was born 4 weeks early.

At 9 month check up he wasn’t crawling. Doctor said it could be because he is a big boy (99th percentile for weight and 98th for length) so it could be tougher for him to do some things. Now that it’s 11 months I’m getting worried.

He can sit unsupported but cannot get into a sitting position on his own.

He army crawls but doesn’t crawl with belly off ground.

He doesn’t pull to stand at all but can stand while holding something if we put him in the position.

Should I be concerned and start wondering about early intervention? Not sure if it’s relevant but he was born with a heart condition (SVT) and has had to take 2 different heart meds three times a day since birth.