Personaly, I would tell him. It is a way of showing that you believer there should be no secrets in M. By "showing by example", at least he knows that you are not hiding anything.
Now, I would be very careful about how/when to bring this up. If you make it too "official", it may seems like there is something going on when at least you know you are not. Has he ever expressed the fact that he does not like you going out with him? If so, then telling him in a more official way may be OK because you know he does not like it but you are doing it anyway. But if he never said/showed anything explicitely, then I feel somehow picking an opportunity while you two are chit chatting to tell him you are going out to lunch with so and so. then see how he reacts. I would not bring up anything like "see, you are hiding things, I am not." or any issues with OW contact. Simply you are going out to lunch with a friend, but it just happens that it's this male friend this time. Nothing special.
This is just my opinion. My H seems a bit more nervious now when I go out with my ex-coworkers (who are mostly male). Before he could care less. Now he asks questions like "is he married? how old?" type of questions. It's funny to see his actions at times.
