2000 years and still sane
My theory is that, between the high soil pressure and the lack of air for his usual first deep breath, Jack didn't returned properly in life between the deaths. He just kept dying and dying, perpetually regenerating himself in a sorta of unconscious state, and this is why, when they finally found him, he is overstressed but not ravening crazy or aged.
I don't have a proper explanation for the clothes, though.
Even theorizing that they gave him some of the past-Jack's spare clothes (with a 40 years earlier stile, and very dirty) I surely doubt that Jack had a spare coat.
Luckily TW, so far, is the only show that can stretch my Suspension Of Disbelief above its limits, making me enjoy it despite the various plot black-holes :)