He's 15, set him in the right path by showing him where hard work will get him.
Tell him to find a job, save up money, and when he's ready to dedicate himself to it, pay off his PPL alone. If he can get all the way past that, help with his commercial license.
Or just send him this way, he'll read how depressing the industry is and he may run back to your arms.
I highly doubt he'll manage to save up money to pay off his PPL, he's growing and realizing women exist, his funds will be spent elsewhere.
My sister was in the same boat, but not with aviation. She had a dream and everyone found it to be naive, but after a ton of hard work and dedication, she's just made it into vet school. I'm not saying his dream is naive, but it is extremely hard to accomplish in this day and age and live a reasonable life afterwards, so let him do the work.