Should I email him.once in a while just to let him know that i always care for him?

It depends on your personal situation. Do you think he needs your support and that it's important he knows he can talk to you? If you think that he needs to know you care, maybe because he's feeling alone or unloved, then sure, let him know. But don't do it just because you want to keep reaching out to him so that he doesn't forget about you. What I mean is, make sure that if you do, you're doing it for the right reasons. If you're emailing him because you want to, don't pretend like it's for him instead.

I had a "friend" and the boundaries to our relationship were a little/very blurred. I loved talking to him and sought emotional support from him but I was already seeing someone so I decided to choose my toxic relationship over him. Eventually I missed him and emailed him but after that it just got messier and he started expecting a relationship with me. Eventually it did not work out and after that I decided not to get in touch with him for our own good. So whether to email him or not depends on what are you expecting after emailing him. If it brings peace to you than why not but if it's going to create complexities than it's better to avoid it.