My 6 month wont sit up, what can i do to help him?

I try and try to make him sit up , but all he wants to do is stand up . I got him a bumbo seat but he wont sit in it either he stiffens his legs out and can about get out of it , well i think he can . also I play with him in the floor alot and i also got him a jumproo thing and he love it , he dont crawl yet either, but he can roll when i lay him on his back he rises his head way off the floor , i just dont know what to do b/c i cant get him to sit down a min without him tryin to stand up !! so plz help me thank you