Should I tell my boyfriend? Is this like cheating?

if your boyfriend was talking to an ask would you like to know? that's what you should realize

No its not cheating. And if you feel that you should so you don't get his hopes up of starting a relationship then yeah go for it, but if you know for a fact that he doesn't want to get together with you then don't worry about it.

But think, if your boyfriend was talking to his ex and she didn't know about you how would you feel?

i think you should tell him because if he was doing that you wouldn't like that.
and I don't think its cheating. but you have to realize if he gets mad at you, its ok if he does. he's your ex and you obviously had a history together and he doesn't want you to go back with him.