Here’s an ultralight pack list for $500 (including clothing and consumables). A lot of it he’ll already own. Keep in mind that it leans heavily toward cheap and relies a lot on finding used gear. This example is a tarp setup without a bugnet. It’s pretty old; notice that nothing was bought on Amazon (where you can get a legitimate nylon tarp for like $45 instead of a blue hardware store tarp for $18). Still, it should give some ideas. And it’s pretty well organized.
https://lytw8.com/gear-lists#5c469117-75ff-4a85-8705-e8486c51c463 