Mystery horse.. EMS? Cushings?

We have a 5 year old arabian gelding at the farm where I work who just doesn't seem quite right to me.

If he were aged I'd probably put my money on cushings but he's 5 and has exhibited these "symptoms" for as long as I can remember ( he was bred and born on the farm)

Excessive drinking
Excessive urination
Sweats in random patches on his body
Always seems agitated and his performance under saddle is declining because of his attitude, he's very fussy and always a bit of a grump.
February of 2011 he foundered after a long haul cross country to a horse show, since then he has had bouts of lameness with heat and pulses in his feet.
He's not overweight but is an easy keeper

I lunged him today (he's usually worked by the trainer) for the first time in a long time and was surprised by how sluggish he was. He's a pretty hot english pleasure horse but he just looked stiff, slow and lazy today.

Some of these things may be unrelated but his excessive drinking and urination ( his heavily bedded stall needs to be stripped daily ) have always made me wonder; would love to hear some thoughts!

He may be Insulin Resistant.

http://www.thehorse.com/pdf/factshee...resistance.pdf

Yes, I'd treat him as if he were IR & possibly get a Cushings test done. It's not a disease that young horses commonly suffer from, but going from memory, I think the age it becomes more common is about 7-9yo.