To be honest it's only just occurred to me this might not be right... is there any cause for concern? I only started thinking about it because he seems to have just started trying to sit up on his own but hasn't managed it yet. IE if I put him sitting he can sit very well and has done for ages, but has never got himself into that position.
Likewise he has been standing for ages wanting us to pull him up, and is now walking pretty well, but doesn't pull up on things.