Gay guys please answer from your experiences. Did this guy change or is it a phase?

dating a guy Ive known for a few years, hes always been promiscuous, right up till 6 mos b4 we started dating. Since we been a couple hes been faithful as far as I know. Says that when hes in a relationship he believes in monogamy and that he wants a serious relationship with me. Hes 37, so Im wondering if its just a phase because hes trying to get me or do guys really change their ways?