IEM's are tremendously killer at blocking out the unwanted mix of all those blasting stage volume sources.
The singer can't hear himself through his bones. Notice how many live singers (without IEM's) need to put a finger over their ears?
You may need to mix mainly aux fed vocals-only into his ears...This is a direct connect, with maximum SPL reduction, minimal source bleed from the stage. Just like foam earplugs, but with a very high fidelity speaker cutting through, AND...At a very comfortable low volume. (Or very high volume too, if you want.)
I use IEM's, but I use a homebrew setup consisting of the Shure E2's earpiece, that sounds VERY good and loud. But I skimped and bought a $50.00 Jensen analog tuned stereo transmitter/receiver pair with body pack Best Buy. Been doing this for over a year or so. It's OK for us for now. I don't move around much sitting on my drum throne. Transmitter/receiver pair is usually within 5 feet, with broadband (wide tolerance) tuning, but I've used it reliably for 20 to 30 feet, where as tuning gets more critical.
Problem is, If anyone else in our band wants to use IEM's, which they DO now...Then we MUST empty our pockets and get a killer state-of-the-art multi-channel system.
The only thing we'd consider for multi-channel use is the Shure PSM systems. ONLY because of their price range, "name", and availability. It is easily obtainable at many MI stores. Still very expensive though.
Happy searching. Lots to choose from.