We've always had a dog flap, they are such godsends! Try holding it open and have someone on the otherside calling him then lots of praise and a treat and keep doing this until he gets the hang of it, then slightly lower the flap and do the same. Gradually lower the flap so he is starting to push it himself. Be patient some dogs do take while but he will get it eventually. My friends dog took weeks then he would go out but not in, but he soon worked it out when he got cold! Luckily my dogs picked it very quickly and I wouldnt be without one now.