I think the best way to do this is to be as direct and as clear as possible while remaining polite. That is, to not give any opening for a rejoinder or misunderstanding of mixed signals, etc.
You think he’s a nice guy (and this works even if you think he’s not so nice.) Then just tell him, “Jim, I think you’re a nice guy, but I don’t like where this relationship could be headed. I don’t want there to be any possibility of coming between you and your wife.” Period. If he is a nice guy then he will understand, without you having to also spell it out for him (but you could, even so) that “this extra friendship makes me uncomfortable”. (If he’s not a nice guy but moves off at this point, he might think that he’s got you fooled “Ha! She thinks I’m a nice guy!” and you should not disabuse him of the notion that you have him all figured out. This is where a lot of women go wrong: they’re smarter than the man about a lot of these types of relationship issues… and then feel that they have to prove it. Don’t. Let the creep – if that’s what he is – think that he has you fooled into thinking that he’s a nice guy. What do you care, as long as he leaves you alone?)
If you don’t think he’s such a nice guy, or if you’re just not certain, then: “Jim, you seem like a nice guy [indicating that you haven’t made up your mind yet; you’re still on the fence about this], but I’m just not comfortable being so friendly without your wife present.” Either he is a nice guy, and he gets it and moves off, as before, or he’s not a nice guy… and proves it by ignoring your request. And that’s when you might have to have the chat with HR.
But even if he’s not a nice guy, unless he’s a total out-and-out creep, which remains to be seen, when you give that indication that “I think you might be a nice guy” ... he’ll probably want to prove that he is (or let you go on believing as you claim to, thinking he has put something over on you), and do as you ask.
Using clear, simple words and NO indication of “if you weren’t married…” or “at some other time in our lives…” or “if I were some other kind of girl…” or any other possible “what if” scenarios that could have maybe led to a different outcome, he should shove off with no hurt feelings, and no possible way to claim that you led him on, threw out mixed signals, etc.