This is a really hard situation and I understand that it must be very hard to be in this long distance relationship! I am going to tell you a few things that may be hard to hear, even though I feel for you, and know the temptation you may be facing.
This guy who offered you a "secret" relationship is either going to want more, or he's using you. And even if he doesn't, or isn't, is it fair to use him? Which is what you'd be doing if you played with his heart just to have someone to hold and cuddle while you were in the US.
Another thing, if your boyfriend did find out, imagine how broken his heart would be. What if he told you that he took a girl to his bed because he was just lonely and wanted someone to love and cuddle close to where he is?
I have cheated, and I have to live with that guilt, and it tears me apart, please think and meditate long and hard on this decision. It's not just you that this effects, there are two men that are involved: one you do have deep feelings for, and one you probably will develop deep feelings for.
I don't judge you sweetie, I know what you're facing, and if you want to talk PM me. If I had it to do over again though, I would have chosen the faithful route, because people got hurt that didn't need to be hurt at all, and all because I made a decision based on a temporary loneliness.