This is quite similar to aonther post we had and have been responding to although I cannot find it at present for various reasons which really annoy me!!!!!!
anyway, obviously he shouldn't be let out unsupervised until you are sure he can use the flap - in case he needs to get in quickly.
propping the flap open with something (I use string and tape) to leave it partially open to give a head start.
If it is too cold in this weather to do this, then get a big box , cut a flap in one side and play with him until he gets the idea in the warmth of your living room - or else wait till the spring.