The Art of Manliness also has some good stuff including a decent podcast and a club type thing you can join. They say it's like Boy Scouts for men, but I haven't done it so I'm not sure.
There are helpful books that devote chapters to different aspects of becoming an adult -- budgeting, planning ahead for life goals, learning life skills, household tasks, etc. One such book that's helpful to have around to refer to is Adulting 101. It includes a section in the back devoted to survival and thriving in college.
https://www.amazon.com/Manual-Manhood-Perfect-Impress-Survive/dp/0800722299 This is the best thing you can get him.
You can search "gifts for 15 year old boys" into amazon and it comes up with some great stuff, such as this masterpiece.
Lol this - https://www.amazon.com/Chicken-Soup-Teenage-Soul-Learning/dp/162361046X
I got a copy of that way back in the day and there was a copy of the Wear Sunscreen speech in there. Not sure who it was attributed to, might be Mary Schmich.
Fr. Mike Schmitz has an awesome(!) 40-page booklet on everything you need to know about vocation/discernment. Your priority should be to grow in holiness each day through prayer and detachment from sin and inordinate affections [retreats are great for this, though not indispensable]. Only when the waters of your soul are clear enough you'll be able to distinguish those desires that are merely superfluous and artificial (ex. buying a Rolex, having a supermodel girlfriend to show off) from those springing from the very depth of your heart. You'll thus be able to discern (with the help of a spiritual director, ideally) whether the vocation Christ has put in your soul is a call to priestly self-surrender in service for His Bride the Church, or a call to be the symbol of God's presence for a woman in your future family.
Read How to Make Great Decisions by Fr. Mike Schmitz. It's an awesome 40-page introduction to discernment and vocations.
Some Protestant churches really push their people to evangelize. It can also be very dangerous for the Christian young person to just approach random people.
One time I was reading the Amazon reviews for a religious book written by, I think, Alex and Brett Harris. They are the brothers of the guy who wrote I Kissed Dating Goodbye.
In one review for the book, a girl wrote that she knew she had to evangelize so she approached three big men who looked scary to her. Inspired by the Harris brothers' book, she completely ignored her own instincts. Luckily for her, the three men were nice to her She came away with the idea that we should all just ignore our fears and go up to complete strangers to tell them about god. No, do not do that! That's dangerous and could get someone killed.
I can't find that particular review but here is the book, Do Hard Things and yes, it sounds like a porn title
https://www.amazon.com/Do-Hard-Things-Rebellion-Expectations/dp/1601428294
Chicken Soup for the Teenage Soul on Tough Stuff: Stories of Tough Times and Lessons Learned.
Do Hard Things is a Christian book, though I’m sure this sticker is more of a nod to her life than this book. Just thought I’d throw this reference out there.
As kind of a gag gift you could get him the Welcome to manhood book (Which most importantly includes how to impress girls ;)