Boy, have we been there! Our children are over 3 now and one reacts to wheat, all grains (including oats and rice, dairy, eggs etc. The other one is mainly just nuts, seeds and dairy. So birthday parties are nightmares. However, we've come up with lot of alternatives to the traditional wheat-based birthday cake. (the first year i think i simply made a birthday cake and he blew out the candle but didn't eat any!)
For frosting, you can use maple syrup or powdered sugar and water or if he's ok w/ soy, the Earth Balance buttery spread is a really good alternative. or, Cherrybrook Kitchen makes a dairy-free frosting we used recently (dairy free) that is great. Their cake mixes are not wheat free tho.
For a cake this year we made a buckwheat maple banana cake. It was pretty good. quite dense. and more like banana bread than cake but it did the trick. We made it in the shape of a train and decorated it with some colored frosting. and everyone ate it w/o choking it down too much! Our son doesn't know the difference, so he loved it. We got that recipe from the Allergy Self-Help cookbook by Marjorie Hurt Jones. It has a bunch or recipes that are free of ALL the common food allergens. Happy birthday to your little one!