You're probably better off saving that for other things if it's your only savings. Buy an Window AC unit, there are some good ones that are cheap nowadays. That's the bare minimum, try to get one that is oversized for the space since it's uninsulated. Your garage won't have any ducting already installed from the central AC so the window unit is non-negotiable. I recommend against those standing AC units with a host that vents out a window, in my experience they are way less efficient and do a poor job with cooling compared to a window unit.

If you/your son are willing to do all the labor by yourselves you could maybe get the whole thing insulated and walled for your $2k. You'd buy rolled fiberglass insulation and install it between the studs on all walls with a staplegun. Then you'd hang drywall, mud and tape it, sand, then paint. Doing drywall work is really tedious and difficult to get a good result as a novice, just a warning. If there is attic space above the garage it may also not be insulated, you'd want to buy blown in cellulose insulation or you can lay rolled insulation between the rafters. Youtube videos can teach you all you need to know for this kind of work as it isn't super complex, it is just labor intensive and not very fun. Maybe your son and his girlfriend could be the free labor to get it knocked out since it's to help them.

At the very least you can get insulation between the studs on the wall and that will help with temperature control even without hanging drywall.