I have been trying to convince my son to get up off his tummy for months now. He is absolutely resolute that pulling himself along on his belly and crossing his ankles so that he only ever has his right foot on the floor is the only way to go. I have tried rocking him, geting him on all fours etc. but the moment I stop holding him in that position, he slides straight back onto his belly. I think it may be in part to his cloth nappies, but even when he is bare bottomed or in disposables he shows zero interest in getting up off the floor.
According to my mother, not one of the five of us crawled 'properly' and the opinion in my family is that once babies have found a way that works for them, they generally aren't interested in further refinements!
I am also concerned that he is not developing any kind of cross-pattern anything, pulling forward simultaneously with both arms and using only one foot, but I can't convince him otherwise and have been trying nearly six months! He is not even pulling up on anything! We have an appointment with his physiotherapist in a couple of weeks (he gets checks every few months because he was premature) and I'll let you know if she has any advice to get him on his hands and knees.
I wouldn't worry too much. Encourage him but don't stress if he isn't doing is (your stress will only upset him and make him less likely to succeed). If he wants to, he will. Probably when you least suspect it