Good for you AND him. He must thnk very highly of you to spend a decent amount of cash (at 15 yrs old) for such a purchase. Even if he did ONLY buy you a bass just to accompany him (which I don't believe), he is clearly dedicated to his drumming enough that he would invest his money, not in new gear for hmself, but in new gear for someone else. That means that new bass CAN'T live in the closet or in its case, even a year from now.
Welcome to TB as you'll find tons of additional support for your new passion.