Have you tried setting aside a night every few weeks as a special date night? Maybe cook him dinner, give him a back rub, put on something sexy that you know he likes, & take it from there.

When your partner withholds intimacy it's usually because something serious is going on within them. It doesn't necessarily mean they are cheating, but he's definitely got something going on in his life. I've been in a few relationships where towards the end I withheld sex & it was because I was unhappy due to feelings of resentment for various reasons. I suggest you sit him down for a serious talk before assuming the worst. You never know how he answers.