Just my 2-cents.

Buy a pistol that you think he would be proud to own for all his life. Then give it to him when he is legal to own it.

I was with my grandfather in an old Gibson's department store when he spied a Canadian Centennial Winchester Model - 94. A few years later he gave it to me as my first deer rifle. I dind't know it at the time, but he bought it with the intent of it being mine. I still treasure it and think of him every time I pick it up.