Practice is the answer! I couldn't hear the beat very well when I started dancing and I've gotten much better over time. For me, counting the beat doesn't work as well as moving to it (I guess the part of my brain that deals with music and the part that forms words don't communicate very well). Have him practice tapping out the beat with his hands, nodding his head, or stepping in place to the beat. At the beginning, I would sometimes tell my instructor I couldn't hear the beat and he would point out that I was moving my head to it anyway. He can also practice any time the radio is one. At stop lights, I always tap the steering wheel to the beat of whatever song is playing (ballroom CDs would be the best for practice).
If he doesn't have much musical background, he might have trouble knowing what part of the music to listen to for the beat, especially if there is no percussion or a lot of other percussion besides the beat. If that's the issue, listen with him and point out which instruments are carrying the beat in a particular song.
He will get better with practice!