First... KUDO's to the kid for earning $4k working a summer job.
I would recommend that he start out with only what he needs to get started, and spend and add what he needs when he is ready to move to the next stage..... he will know when that happens.
The temptation to spend the $4k all at once to get "the ultimate bedroom studio" is very strong. But he should resist it.
Since he has a laptop, a guitar and a small amp..... here's what I would recommend.
1. MC6 $40 or X1/2 essential $100?
2. a decent USB interface $250
3. a POD for the guitar $300
4. nice set of studio monitors 5" cones $350/pr
5. sub woofer $300 to $400
6. head phones... between $100 to $200
7. good condensor mic $400 or less
All that will set him back around $1500 to $1800 give or take. It will get him started and allow him to learn the basics and do some really nice recordings based on his experience level.
Beyond that... as the need emerges, drum software and other things to make the recordings sound better can be bought.....
I do not recommend that he walk into a store and announce that he wants to set up a studio and has $4000 to spend to do it..... they will sell him $4k worth of stuff....and he may not need it. With the list above he gets started and only spends half of what he saved. I think that is a reasonable way to get started. Heck I got started for much less than that and added what I needed as I realized I needed a better way to do something.