^^Agreed, if typical AAA games and AH3 are going to yours/your sons primary deal, 1060 will do the trick and then some IMO. If $ isn't a big deal, by all means, a 1080 will give you more performance, no doubt, that CPU will drive a 1080 just fine, again IMO.
My experience with both the Vive and Rift when I briefly tried a 1070 was OK, but the 1080s I had were a big improvement in VR, and the 1080ti/Titan are an improvement on the 1080 too in the VR department.
Personally, if VR isn't something you're going to be doing anytime soon, and with a system that is older like the one in the OP, I'd just buy a good used 1060.