Dell will only reply with cards they have tested as compatible, and that list is fairly limited.
The XPS 8700 has been shwn to work with a wide variety of GTX graphics cards.
In our family we have had GTX 760, GTX 770, and GTX 970 cards in our 8700s.
Here is a thread with hundreds of posts showing what others ahve done
The key things to succeed are:
1) BIOS A10 is required for an 8700 to work with any of of the GTX 9x0 or 10x0 cards.
BIOS A11 if you have upgraded to Windoze 10.
2) the Stock Power Supply limits which cards can go in with out a power supply upgrade.
3) Budget tends to drive the final choices.
Normally I'd ask what resolution your son has (1920x1080 is most common) and what games, but I'll just toss out a suggestion to start off looking at a GTX 1060, 3GB.
Thes cards run around $220. Last week one was on sale for $180.
The stock Dell PSU has enough power to run a 1060. Some 1060 cards have a single 8 pin power connector, so you will need a $5 adapter that takes two six pin inputs and gives a single 8 pin output to power the card.
If this is out of your price range, the 1050 and 1050ti cards might fit the bill, but I would lean towards the 1060 as it will handle more games and stay relevant longer than a 950 or a 1050.