I would get his favorite color sweat suit then go somewhere like Hobby Lobby or your local craft store and get two pieces of some type of thick foam board, or heavy poster board. Cut out a two huge number fives. Punch holes in the top and in the slides then have him slip it over his head and put his arms through it. Am I making any sense. You could then paint a number 5 on each check on his face.
Maybe you could take two peices of cardboard, tall enough to cover from his shoulders to his thighs/knees. Lay two peices down, draw a chunky number five on the top one, cut both out to the pattern, and use string or something to make an apron like thingy (the style that people use when they wear signs for picketing things) to go over his head. He could have fun painting it whatever color he wanted, putting designs on it as such.<br><br>
You might even be able to find free boxes you could cut them out of at an appliance store or a walmart (especially in a 24 hour store...if you show up at 4-5 am there are always tons of emply boxes everywhere)<br><br>
What a cute & original idea for halloween!