Hi,
I have been in a monotonous (whoops I mean monogamous) relationship for 38 years with a little swinging in the first year or so. It made me realize that I didn't have the right kind of constitution for it. Why I bring this up is because I consider some of the decisions I made mistakes; like steering my wife into my desires when in actuality I didn't enjoy her doing it with another man. That's my backround, mostly monogamous.
So anyway, I think if your relationship is only a month old you should do what you feel you want to do. I see your point of view clearly and understand after inflicting wounds upon myself with bad judgement. Your right your guy will hate it. You are still young and could have made a mistake. If you don't want to tell him don't.
Other than that I agree with the others about trust. If this is a one time lesson so be it. If you do it again then you are going to cause real problems for you and your partner.
O.H.