You don't have to tell him right away. I'd wait it out. It depends on how much you know him too. Like do you talk to him and see him a lot alone. If you're not comfortable saying it now just wait. I had a facial blushing problem (got rid of most it it from ETS) and in the past, I would talk about it after I felt that the person I liked or dated would be supportive. I mean you might still not know, but just look for signs, if he's into you a lot too it will be easier. I said something like "ahh my face is so hot" (when I felt a blush coming on) "it's this condition I have...I have this facial blushing problem...my face gets red often" etc.. and continue the conversation.
You could probably just ease into yours. Like if it's a hot day, just say "it's really hot out"...I sweat kind of easily...I found out that I'm an excessive sweater...etc... Just say it in your own manner. Just wait until you're ready. Don't put to much pressure on it.