How many days are you thinking? I was used to that kind of schedule as well and my DS on his first trip got sick the 4th day after breakfast. All it turned out to be was that we were going too strong and he ran out of steam. Took most of the 4th day off as a result, and he was back full force for the 5th day. That trip, he almost never made it all the way to closing. We only stayed to closing one night. We left once he got tired the other days (Which took some interperting on our part cause he will go 100% and then just zonk out).
Since this is his 1st time and he is 5 (Same age my DS was his 1st time), I'd recommend taking it a bit easy.
Also, since your GF is new as well, try not to be too much of a tour guide unless she asks you to. Let her and her son lead the exploration. My DW never went to a disney park until I took her to WDW right before we got married and I had always gone clockwise in DL (The way the old souviner guides were laid out) so I tended to want to lead that way and she wanted to just explore so that took a little change in my old habbits before we were both in sync. Best to let the newbies do what they want to and adjust your old habbits to it. Eventually, everyone will be on board with the flow and it will be fun for all.