J.S.
I'm not sure about the ticketmaster discounts, but I can give you a couple money saving tips for in the park.
--DO NOT pay for preferred parking if its offered. Its about double the cost and depending on when you get to the park, you may be just as far away as normal parking, but left without the shuttle they provide for the people that park in the regular section. Plus, you will love the shuttle when you leave the park and your car seems 10 miles away. :)
--Pack a cooler for lunch. Leave it in your car with a couple beach towels or blankets. You can go back to your car, grab the cooler and sit at a pavilion on grassy area in the parking lot to eat. Lunch and drinks for a family of 4 can cost well over $50 in the park. They will stamp your hand at the gate so you can get back into the park after lunch. It's a nice break from all the chaos of the park. The last time I was there, there was a pavilion right by Apollo's Chariot, your son will think its cool to watch the roller coasters and hear the people scream. :)
--Don't go into EVERY gift shop. Tell your son that he can pick one or two shops to go in and pick one or two items to remember his birthday. (Depending on your budget)
Remember that its not the amount of money you spend on him or the day, its the fact that he is getting to do what he wants for his birthday. Have fun!!