I'd let him know. If I started talking to a guy I'd dated in the relatively recent past, out of the blue, and it bugged Jason...I'd want to know about it. I would NOT hold it against him, especially if he said something along the lines of, "I know it's irrational, and it's not really a jealousy thing, but it kind of makes me uncomfortable blah blah." (Not that it would actually be irrational to be bugged by that but it would be nice for there to be a gesture of "this is more about me than it is about what I think YOU are going to DO with THAT PERSON." Especially since "It's not that I don't trust you" sounds, to me, like "I totally don't trust you.)