How much better is the 285 than the 270X? If it's a big jump, then he might want it, but there are a limited number of graphically intensive recent games that PEGI/ the BBFC think he's old enough to play, so the extra power isn't hugely necessary if it's not a huge difference.
Also, when I picked out his PSU, I did it with a 750 Ti in mind as the most likely thing he'd get, so there's already not a huge amount of headroom left for the more hungry 270X.
What psu did you get him and what are the other specs? I have a hard time believing that unless you really cheaped out big time.
The 285 costs only 10 pounds more, consumes 10 watts more (190w vs 180w), yet performs 1/4 better and is based on a newer architecture with more features (GCN 1.3 vs GCN 1.2). It's a better bang for your buck even if he doesn't need the extra performance.
That's more than enough power. Just do the math (or use a power calculator to do the math for you).
I did a few, and got some to go as high as 429W (although for those I had to approximate a few things as more Watty things). That seemed a little close to the 430W to me, but I also didn't get anything back saying "This is assuming you're using an efficient PSU because you'll need this much power from it exactly". You know more about this than I do, though, so I'll trust you.
Gimme specs. I don't believe you unless you've got like 20 HDD in there.
The top one on Google:
http://www.extreme.outervision.com/psucalculatorlite.jsp gave me recommended 429W when I told it I'd be OCing to 4.5 GHz with 1.21V, had a high-end desktop motherboard (it's the cheapest Z97 Scan had when we bought it, but Z97 is higher-end than low-end), 1 SATA 7200RPM drive, a DVD burner, 2 120mm fans, a r9 280 (as it didn't have the 285 as an option) and everything else as default. However, I now see that the number in big letters is a recommended wattage, not a minimum, so it's already applied a safe headroom.