7 month old

I know this is something I should talk to my pediatrician about and I will but I’d like advice from other mothers as well. My 7 month old cannot sit on his own or even with assistance. He just flops down. He’s starting to crawl by scooting on his belly and he can flip, he just cant sit up. My other son could sit just fine by this age and I know all kids are different but is this a cause for concern or he’s still within the normal age to be a little behind?