Sitting up trouble

My son is eight months old and he is right on track with all of his milestones except sitting up by himself. He can sit up excellent if he is being supported. When I try to sit him up by himself he cannot balance himself. He’s very strong so I know that’s not the problem it’s just that he cannot balance with him self. All day long he loves to be on his belly and crawl around the house. He’s just not interested in sitting up. Being a first time mom I do getting worried why he hasn’t been able to set up alone yet. Has anybody else experienced this with their babies?

What can I do to help him