On sitting up, none of my babies has done sit-ups. They use their arms to push themselves up. I always thought that was normal? I actually remember reading that our babies have been developing abnormally strong abdominals since the back to sleep campaign started, coupled with babies being carried all over the place in car-seats.
Yeah my older 2 learned how to crawl before they learned how to sit up. Once they could crawl, pushing up into a sitting position naturally followed. They never did crunches to sit up. Avery does try to sit up that way and I think it's b/c he doesn't understand rolling over and pushing up yet.
Becky, maybe you could get him evaluated for low muscle tone and see if there is anything to be worried about. Usually this is covered by insurance as well as a few PT sessions that might help him out if there truly is an issue. It could put your mind at ease. Though I don't think you have anything to worry about for several more months.