DC 19 is really high for that level.
10-base
04-Int bonus
01-Spell Casting Prodigy
01-Spell Focus (Conjuration)
01-Greater Spell Focus (Conjuration)
02-Level of the Spell
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19-Total DC
That's a lot of investment he's put into the Conjuration school, and there should be some reward for that.
I'll be the first to admit that Glitterdust is utterly broken, and imo, one of the most powerful spells in the game. My DM mitigated it by making monsters with increased Will saves, Globe of Invulnerability, and my favorite, NPCs who wore goggles. When they got Glitterdusted, they simply flipped the goggles up to their foreheads and continued fighting. I still disagree with that, but what are you going to do?
The other issue is that this is not a temporary problem. When he gets Heightened Spell, he can jack the DC of Glitterdust up to his highest level of spells. Sure, he loses the 4th or 5th level slot for the day, but most of those spells don't even have the raw power of Glitterdust. On my wiz I'm using Heightened Glitterdust for 2 of my 6th level spells, and it works like a charm.
In any case, good luck on fixing this. Some have suggested to change it to a Ref save, and I agree with this approach, though it really doesn't help an incredible amount. Our current DM hates the spell and is considering outright banning it, but I don't think this is the best approach. Your fix needs to have subtlety and not be a universal ban hammer.
Good luck,
Krensus