Our A/C for the house is set to 76 "cool". Outside is 74 and will drop through the evening. Our son likes his bedroom cool so he opens his window and shuts his bedroom door. Will his window being open and his room cooler make the house A/C work more?

The problem is, the house a/c being set to turn on when it warms up to 76 inside we're concerned that with his window open it is wasting energy... is it?

Yes and No, If the window is open and the air is coming into his room, that air is being wasted. Nothing wrong with what he is doing, but I would suggest that you shut off the air input to his room if you are going to run it that way. Shutting off his room is actually going to give you better air conditioning in the rest of the house. Others may say that is wrong but I think it is right to do.