Sex Advice (TMI)

Husband and I have been married 3 years this July! But when I met him he was new to sex so it didnt surprise me when we had to learn things in the beginning.... fast forward to now we have a 1-year-old and a great sex life since I got off birth control recently but there is one thing that has been bothering me...**TMI** He has a VERY hard time inserting his penis from behind. I'm average height for a woman (5'6) he is a little on the short side at (5'7ish)and I want to say hes average down there to...(he thinks it's because he's smaller/shorter). Its like laying down sideways, doggy-style, standing up and any kind of behind sexual positions are hard to impossible for him. I've been with "average-sized" shorter men before so I know it shouldn't be this difficult to do. I just dont know how to help him fix it. (Also he is uncut not that that has anything to do with it but he is a lot more careful because sometimes it hurts him if the skin pulls a wrong way and he says the angles dont feel right!!) Please help if you can, thanks ladies. Sorry if we sound so inexperienced, you would think we would have gotten this down in 3+ years together but still a work in progress..