If the guy calls, my advice is to have just a normal conversation with him and throw in a little comment about your boyfriend in the middle. He'll probably get the hint.
For example: if you're talking about cool places in town to hang out, mention that you and your boyfriend are going to dinner at one of the popular restaurants on Saturday.... or if sports come up in the convo, say "oh yeah, my boyfriend plays intramurals for XYZ." See what I mean? Just a casual statement will let the guy know that you're off the market, but it won't sound as if you're trying to dismiss him from your life completely. Hope this helps!
