Having owned both, I'd say that most significant difference between them is the build, rather than the sound (aside from price, of course!).
I
did hear differences in the sound, in line with what most people report (ie 225i = darker, more laid back, 325i = more aggressive, shriller highs), but they're pretty slight and I doubt most people would notice if they're just listening to music and not obsessively comparing the headphones
What pads does your son use with the SR60? If he uses the stock S Cushion (comfies) I'd say he'd be pleased with the upgrade to the 225i, as the just having different pads will bring a significant sound improvement.
If he already uses the L Cushion (bowls) then I'd say go for the 325i. The difference in sound quality wont be enormous, but the metal cups & leather headband will make them seem like a real step up.