H.M.
I wouldn't be concerned. He is still young and like you said everyone develops at different levels. My son is 15 months old and didn't start crawling until he was 13 months old. He is just now getting to where he will sit up on his knees to look at something but still won't pull himself up to a standing position. It is discouraging when you see the other kids in their daycare class that are doing things earlier but they will do it when they are ready.