I have my brothers S60R out here for a week and the check engine light is on. I want to figure out what it is for him without dealing with volvo dealers.

for a volvo!

Did you put any fuel in it yet? If so there's a good chance the gas cap isn't put on right. The P2 Volvos in particular are sensitive to this and it's easy to put the cap on cross-threaded. There's plenty of posts on Swedespeed about this and it is also quite common to have this happen in places where full-serve gas stations are the norm (e.g., New Jersey.)

If it is the gas cap, take it off and reseat it. The idiot light will disappear with a few days of driving if this is the cause (the OBDII sub-system for gas vapor removal does a reset essentially as I understand it.)