Boys who dance get a LOT of positive encouragement and attention from dance studios who really want boys to come into their programs!
Look for a studio that offers (as Dancin Momma said) jazz, tap and hip-hop but I would also say if you find one that has those plus ballet, it indicates a studio where they are really well grounded in the basics, even if your son himself does not take ballet (though he might want to once he sees it up close). See if you can find an all-boys dance class in any form of dance -- that often helps younger boys feel more comfortable but those classes can be hard to find. A studio with at least one male teacher is nice too. Our studio has two men who are teachers for modern and ballet and I know it makes the boys (we have only a few, alas) feel like they have great role models.
Please do have him watch dancing shows. Get some videos of the great tap dancer Savion Glover -- they're out there for rent. Rent a video of the dance show "Stomp" which he will adore. He might even like the dance sequences from Gene Kelly films -- he combined ballet and modern and ballroom and tap styles in a very masculine and graceful way.
A very cheap way to expose your son to dance is to take him to recitals and shows by local dance schools! Most will have spring shows and recitals in May or June -- call around and check the local papers and look for flyers in your area. Take him. You could check in advance and see if there are guys dancing -- if your son sees only girls up there he might not be thrilled but if a program has boys in it he would be inspired.
Guys dance too. I really wish your son could see some of the teenage male dancers I've seen around here, or my daughter's modern dance teacher who is a professional dancer (and as it happens, a guy).
I'll briefly share: My friend's son (now grown) took ballet for several years right iinto middle school. One day he was getting teased by his baseball teammates about ballet. The coach stepped in and said,, "Who has the most stolen bases on this team?" Well, my friend's son did; he was a great stealer. "That's because he can stretch almost from one base to the next one and he can do that because he takes ballet. So unless YOU guys can do that, I never want to hear you mess with him about dance again." A great story for whenever a boy is getting flak for dancing!