Baby shows no interest in crawling at 8 months

My son is 8 months old and he still hates tummy time and doesn't seem to want to crawl at all. Whenever I put him on his stomach he either immediately rolls onto his back or just sits there until he gets upset and starts crying, even with me trying to play with him. He'll laugh at me when I play with him but after a couple minutes he's just done. He also doesn't sit up on his own. Whenever I try to encourage him to sit up he just ends up straightening out his legs and falling backwards. He has a walker and when I put him in there he'll kind of push off of it with his feet, mostly on accident. Should I be worried? Or is he just being lazy? I don't want him to be falling behind 😔