So sorry about your son.
I was "attacked" by a bat in my classroom in Texas in 2003. Make sure he goes to a hospital that knows how to properly deal with the rabies vaccine. The first ER (a small town one) gave me a tetnus and told me to be gone. I called another doctor and he was freaked out by their lack of response.
From what I was told, you have several days to be administered the initial immune globulin. This has a very short shelf life and 3 hospitals in the downtown Houston medical area did not have it. A smaller clinic closer to my home had to have it flown in from the CDC, because again, you are on a time-line to receive it after the initial bite or scrape.
Anyway, I got the immune globulin a few days after the bite, then another set of shots directly into the bite and scrape (on my hand and arm) and a series in my rear. Then for the next month or so I got another 5 shots (hot pink, looked like something out of Weird Science!)
Yes, the shots were painful, and I was very sick with them. I remember my muscles aching tremendously and having bad headaches and even fever. I missed a day of work after each shot because I ached so bad and couldn't move. It felt like the flu a thousand times over.
I'm sorry that your young son is having to go through this. But know that in the end he'll be fine. I'm sure he's a trooper and he'll have a story to tell his buddies about the day the squirrel attacked!!
Btw, one of the students I taught at the time had gone through the rabies shots a year earlier for being scraped by a squirrel, so it is administered to the "squirrel victims" as well. I'd say better safe than sorry.