My friend wants to bring a stranger to my wedding who was not addressed on his invitation...?

I have a problem. My fiance and I are having a very private, intimate ceremony, mostly due to our small budget, but also because we believe that our nuptials should be witnessed only by those close to our hearts. I invited one of my best friends whom I have been very close to since grade school, Kel, and his boyfriend whom I have never met, but they have been together for 3 years now so I didn't think it was fair to not invite him also. I was also really excited to meet his significant other. On the invitation I wrote only their full names. My problem is, I got back Kel's rsvp card in the mail the other day, and he wrote that he would be attending, but that his boyfriend cannot make the trip. Instead he said that he is bringing a guest who is one of his friends, but whom I have only met once in ping at a coffee shop. My fiance and I are not comfortable with Kel bringing a stranger, who is just a friend to him, to our private wedding ceremony. I also do not feel that we should have to feed this person at my reception when we couldn't even afford to invite our third cousins. I don't want to seem rude, but with all of the obstacles we have had to overcome just to have this wedding (being downsized from my company, starting a new career, totaling our only vehicle two weeks ago, paying for EVERYTHING completely by ourselves) I would think Kel would not think twice to bring anyone other than his signifigant other, but maybe he thinks that it is appropriate. Since two people were invited on his invite in the first place, maybe he thinks it doesn't matter who fills the second spot. Am I wrong if I tell him that only the people on the invitation are invited and that if his boyfriend can not make it then he should come alone? How should I approach this situation without offending him or the coffee shop friend since he probably already has been invited by Kel.