what to buy for a 17 year old?

As a recent(ish) 17 year old, good things are always gas money and free food. A gas gift card or fast food gift cards is always great, and another thing that I appreciated was having a portable bluetooth speaker. He may already have one or not be interested in it, but it's nice to have one for when you're hanging out with friends or in the garage or doing something where headphones aren't practical. Just some suggestions!