uuuuugggg really? I think in this day and age it's okay to casually talk to multiple people? Really, who hasn't done that? It's okay to talk casually to different people, be friends, hang out, text, flirt; it's nothing harmful. It becomes a problem when you're intimate with multiple people, start doing things like going on dates, spending multiple hours together alone, kissing and touching and flirting more intimately.
He's a grown man, he's not committed to anyone, I hope he wasn't intimate or dating anyone during that time, it's okay for him to talk casually with different people. When I started talking to my bf, I was talking to another guy also, but we only texted and fb'ed as friends (well, more so than my regular friends cause we knew we had some attraction to each other). Once I started going on dates with my bf, I stopped talking to the other guy to get to "know" him more than a guy. Same thing about my bf, he was actually talking to more girls, but once he wanted to be more serious with me, he stopped responding as much. You just let the communication die down slowly