I'll share a few (perhaps offbeat) considerations, as I've worked at daycares and we had birthdays at my preschool:
Please talk to the teachers first. As was earlier suggested, the children need to have things pretty routine during their day, so talk to the teachers about when in the day a party might work.
Next, I'd check in about the food. Some kids are lactose intolerant, which means pizza and ice cream won't work for them. Celiac/gluten-free diets? Same thing. Find out how they feel about sweets, too. At my preschool (and many do this) we had a 'no sweets/treats rule' because children do tend to crash on the sugar and then we have a massive classroom meltdown. For many preschools, birthdays are wonderfully celebrated with fruit skewers or some fresh fruit cut up and served in cupcake cups. This makes sure the kids get something nutritious and won't go home cranky.
If you are looking for an inexpensive goody for the boys, try looking at your local bookstore for a Dover mini-books display. The books are usually $2-3 dollars a piece and they have a very wide variety-- mazes, tattoos, stickers,stencils, etc. (I regularly grab these up as they fit in my purse and my son can do the activities at restaurants while we wait for food.)
Once again, have a check-in with the teacher before buying food, etc. They may have some of their own usual classroom traditions for birthdays. Happy birthday to your boy, too!