Get a birthday badge (big!) or sash for him to wear all day. He'll get loads of 'Happy Birthdays'! Maybe fancy dress too (pirate??). Go to see Mickey in the Meet Mickey Mouse house thing in Fantasy Land. See the parades and night time shows....
DD turned 4 when we were in DLP in December. She wore a cinderella dress and a big birthday badge all day. Majority of CMs wished her happy birthday, (security guy at the end of the Village even bowed to our 'Princess'!) she had loads of happy birthdays from the people in the parade (the Mad Hatter spotted her and her badge and came over for a brief chat in the middle of the parade! Oh, stand near the START of the parade route!! Less crowded). We decided her birthday would concentrate on characters so we went to the Meet Mickey house thing and the CM on the door saw her badge, called us over, made a phone call and we were ushered to the front of the queue!! Missed a 30 min queue! Doesn't mean it will always happen, but happened for us!! Didn't bother with Princess Pavillion as we had lunch in Auberge de Centrillion. We didn't order a birthday cake, and I was starting to regret it - the logic was that the cake is SO expansive for basically a shop bought cake, and there being only 3 of us we wouldn't have eaten much, and would we end up eating the left overs the rest of the week?? Anyway, no problems in the end as because we'd told Auberge on booking it was her birthday, they decided to pop a candle in her pudding when it came out!! So that was lovely.
Apparently you can also go to City Hall on his birthday too for a sticker? We missed that out but had a certificate from the Passport office the day before when we set up our annual passes and they noticed it was her birthday!
My advice is don't be scared to TELL everyone it's his birthday - when booking the hotel, restaurants, mention extra 'pixie dust' for his special day..... and wear a badge! It was like a magic pass!! lol
Hope that helps!
C x