We really liked Boma.
However for a birthday celebration for a 6yo boy I will tell you what we did for my 6yo nephew and he LOVED it: Chef Mickey's breakfast and I think dinner would be great too.
They'll give your son a CM's placemat, a Mickey cupcake, they'll do a song in his honor. My nephew uses his placemat to eat on daily, only ate the icing off the cup cake

and loved the napkin waving, monorail running overhead, rowdy atmosphere. Heck he even got to dance with Donald Duck and it seemed there was always some character nearby.
The only downside is your son may forget to eat he's having such a good time. Personally I don't think there is a better place at WDW to celebrate a bday for a 6yo boy. By the way I managed to pack in a pretty good meal into my belly while there too.