My oldest had terrible constipation issues that lasted for years. I feel for you, because I know how frustrating it can be. The wait at our Peds GI is similar to yours. There's just no way to get in to see them sooner. One thing you can try is to ask the office staff to put you on their cancellations list. That way, they'll call you if an appt opens up. You can call them every day or two to see if there is an opening.
You really should try to get him to take the Miralax. It's what our Peds GI prescribed a couple of years ago before it was OTC, and it's probably what your Peds GI will recommend once you get in to see him/her. (I'm pretty sure what the pp recommended is the generic form.) It does wonders once it starts working. Just don't let him know it's in what he's eating or drinking. You can add it to applesauce, pudding, jello...whatever it takes. It does take a few days to begin working, but I'd be surprised if you didn't see results.
I'm assuming you are already limiting dairy, and other constipating foods, and getting as much liquid in to him as possible.
Tara
living a crazy life with 3 boys
I am thinking now
of grief, and of getting past it;
I feel my boots
trying to leave the ground,
I feel my heart
pumping hard. I want
to think again of dangerous and noble things.
I want to be light and frolicsome.
I want to be improbable beautiful and afraid of nothing,
as though I had wings.
~Mary Oliver