This is SO awesome.
My suggestion is don't tell him where you're going...just tell him it's a birthday road trip. Print some clues and at each rest stop give him one. Corny idea...but I did something similar with my son when he was young and HE loved it.
Some ideas for the clues along the way:
We're stopping for a drink and snack
Stick with me, no turning back
Let's stretch our legs and take a rest
To prepare us for the journey's test
I'm stopping now to check the oil
Sure hope the surprise isn't spoiled
I have to stop for gas it seems
Fill the tank up for our team!
As you get near the rink and he starts to likely figure things out...
The perfect gift, I tried to pick it
Here son, hold our GAME 5 TICKETS!!
I'm so lame.