Several of my co-workers have kids around the same age. The one we spend the most time with has two girls. One of her daughters had her 6th birthday last weekend. The theme was Merida from the Disney movie Brave. The birthday girl was dressed like Merida; her sister (same age as Will) was dressed as Belle; and the other guests (4 more little girls) as various princesses or just in cute party dresses. That's right: the princesses and Will wearing his Star Wars hoodie and jeans.
Will doesn't seem too bothered by being the only boy. In fact, he always asks about going to Ava and Hannah's house. He LOVES their play kitchen, and they have a lot of other non-princess or fairy toys to play with as well. Plus, we usually go for birthday or pool parties, so there is always fun food and cupcakes!
(Side note: I didn't realize that fairies are all the rage now for 3-6 year old girls. . . .Now I know.)