Buttons are regular buttons with a safety pin back. The MK ones measure about 3" across and say "It's my Birthday". The Epcot ones are rectangular, about 3x2" and say the same thing, but with a border of flags.
He will absolutely be made to feel special - make sure you take him to Guest Services to get his pin (he doesn't HAVE to be there, but they make a big deal of it.
I made a point to get pins from every park - some are duplicates - you can also get them at Downtown Disney. Since we didn't hit all 4 in one day, for two parks I asked the day before and just said we were celebrating a bday.
On the way from Epcot to MK on the monorail, a passenger noticed DD's pin and had the entire monorail car sing her Happy Birthday - really, fun things happen when you wear the pins.
Lastly, we rode in the front car of the monorail with the driver - sometimes you have to wait for the next car, but just ask the attendant when you get there. It's really a blast and a great birthday memory.