My dds did this in October and there were three or four boys at the party(maybe 16 or 18 kids total, I can't remember). The kids play games (hide and seek, simon says) with the Mad Hatter, so there are plenty of fun activities for both genders! The only problem might be if he is the only boy that day, but if he is younger (and with his sister) he might not care at all.
btw, my girls had a great itme, a big highlight of the trip!