Indeed, guards with high IS do perform better offensively in any type of offence, particularly inside ones. Also of great benefit alongside high IS is a high driving stat, which I'm guessing your PG already has a decent amount of.
If you haven't already pounded a bunch of one on one training, then training him for a season or two in one on one forwards would be a nice way to develop several of his stats, including his IS. Given he was an U21 player, I'm guessing he got a fair bit of one on one guards however, so I couldn't recommend any more one on one without first making sure it wouldn't throw his JS/HA/DR too high and out of balance.
If you wanted more specific guidance or advice, you could get in touch with the Australian NT manager, mllama, as I'm sure he'd be happy to guide a possible future NT player - or even myself, if you didn't mind showing his stats to one of your division rivals ;)
Ultimately, it looks like you have a very nice player to work with, and hopefully one you can build a successful team around long term - that's what I did and it's worked for me so far :) Good luck!