hearing what you said, i feel compelled to share this with you.
allow me to go on a slight storytelling mode:

when i was young, my late dad told me: you don't blame your equipment, but find ways in which you can improve.

up until this date, i took his word to heart. in fact, i've seen quite a few friends of mine, to their own detriment, would rather blame their equipments than their own lack of perseverance. while i don't know what type of competitive gaming your son is going to pursue; i feel the need to recommend you to this channel: Core A Gaming, one such example is this: Getting better at fighting games.

i can say with certainty that a GTX 1060 is enough for games like: Dota 2, Lol, CS GO, Hots (heroes of the storm), even World of tanks with all eyecandies enabled. ok, i'll give your son a little bit more slack, a GTX 1070 would be really-overkill. also, if you can find a chance, give your son a mention about Daigo Umehara's moment 37 parry.

the problem is this:human reaction time is the limiting factor, not GPU framerates. no human can react anywhere as fast. give the video a watch in its entirety.


you can spend the money instead to invest on a really good Monitor with lowest input lag, lowest response time. this will help much more than spending money on a GTX 1080s 1080Tis.