I've gone through three 14 year old sons, and each one was a lot different at 14 than they are now. My opinion is that the notion of buying a bow for him, now, that will "last a lifetime" is too, too optimistic. It seems I'm not alone in that opinion.
My 24 yr old is now on his 3rd bow since he was 14: my 21 yr old is on his second, as is the 19 yr old. Strangely enough, I'm only on my second new bow in that same time.
Work with him to select a bow that fits him today, and let him grow with it. When he's 18, hopefully he's outgrown it, both in his height (affects draw length) and strength (draw weight); and most importantly, in his archery skills.
Just like when many of us were teenagers, "speed rules", but in the archery world it's expressed in feet per minute and not quarter mile drag times.