I love the idea of checking with your local police department to see if they have any programs that help foster a positive relationship between town and law enforcement. I don't know if they have rules about off-duty police appearing in uniform for private functions so you'd have to find out and abide by those, but it's worth asking. Sometimes they'll do programs on bike safety, wearing helmets while skateboarding etc. - around here it's the law, so there's a vested interest on the part of the department. Our local preschools have traveled to the police department (of course, the owner of one is married to a cop, but I don't think that's the only reason!). So dealing with older kids, at least on a "field trip" basis and not necessarily a "party" basis might be feasible. They want to get to these kids before the drugs enter their schools, and of course they are concerned about child predators and so on. This is also the age where they want kids to understand about not pranking with 911 calls and so on. There might well be something in place that you just have to ask about.
Craft stores often have wood plaques in different shapes, so maybe the kids could make their own shield-shaped plaques with wood letters/numbers (to make their own badges? You might get some ideas from a sales assistant. I'll bet you could adapt some traditional party games to police-themed versions. A lot of the cake decorating books show ways to turn cakes into simple vehicles (round cookies such as oreos for tires, for example), so maybe there's something you can concoct for him. You can probably find something in the library or in the stores where they sell bakery items (often those craft stores have a cooking section for themes).
