No he won't forget. Depending on the emotional connection he had with you, if it was deep and serious, that person will come to mind a couple times a week.
If it was nothing serious, he might wonder, but then it'll shrug off. I was left by my girlfriend whos an ex now. I was in love, she always crossed my mind, still does sometimes and I've been with other girls. but that's cause I'm thoughtful and a caring type guy. Was just talking to her a bit ago here and there through emails, she has a boyfriend now. she still loves me she said. I doubt it though, she was a real B_... to me.
but he won't forget you, unless he has short term memory. Once you dated someone, they're always etched into some place in memory if that's all there is left.