Sex with boyfriend who is a virgin

Hi, my boyfriend and I have been together for a while but we have never had sex as the first time we tried to have it, it didn’t go very well and since then we have tried but we are not successful. My boyfriend wants me to initiate it and go on top as I have already had sex but it was a few years ago and it was not very pleasant. Also, he was autism and gets overwhelmed very easily. If anyone can offer any advice on how to improve that would be great thanks