Don't worry. I was going through the same thing. My son didn't sit up on his own until he was 7 months and crawled at 9 months.

I've learned through experience that all babies develop on their own time. My daughter was just plane lazy and didn't care for any of it lol, but of course if you're worried (like I was when she was little) just mention it to his dr at the next appointment.

okay, cause my son is 8m and all he do is throw a fit when he on his stomach... @juliakoren

I wouldn't worry to much about it, my daughter didn't start crawling until she was almost 10 months, she's now two and walks all over the place. She also didn't start walking until about 15 months.