He's used a bunch of stuff over the years...Gibsons mostly, some schallers, I've heard custom's as well.
If you want a classic Shenker type tone, but don't use a wah as a filter like he did for so many years, a Custom in a gibson style instrument is a good choice.
But if you are going for classic UFO era tones...PAF style pup through a cranked 50 watt Marshall NMV, a wah (Vox reissues sound closest for stock pedals) for the filtering and a slight analog delay/slapback. I could nail his tone with my 1987X, Heritage 150 with Duncan 59s and my Vox reissue. He used stock and rivera modded marshalls in the early days of MSG.
On that Unforgiven live album from the late 90's he's def using a PAF style pup, through what sounds like to me DSL/TSL Marshall and def no wah. Sounds like crap, honestly. Def not the Schenker of old.