Look into a digital multimeter if he doesn’t have one (and it’s so distinctive it should be noticeable if he does). This tool allows you to test a wide array of electrical issues on a car and is key to diagnosing engine problems.

Another handy thing is a test light. Very convenient.

Or maybe see if he has a good ratchet. If he is using wrenches he will have to turn a bolt, take the wrench off and move to back to where he started and put it back on and turn again... over and over and over. A good ratchet allows you to just turn, pull back and turn again. It comes with different sizes adapters for different nut sizes, the more the merrier. But it greatly speeds up workflow if he doesn’t have one.

A fuel pressure gauge is also super handy to have.

If you were able to sneak us a couple of photos of his tools we could help a lot in saying what he might like.