I have none of the things he needs (Shining bars of metal & stacked leather). Is he going to go insane? Will his insanity be curable later by providing the materials? Should I wall him and the workshop off from the rest?
The ideal solution would be to acquire them if possible. Metal bars and leather are difficult but not usually impossible to get (unlike shells or glass which sometimes must be traded for).
Metal bars are made by smelting metal ore. Any metal will do- mine some, burn some charcoal at a wood furnace and smelt it. Leather can be made by tanning animal hides at a tanner's shop- have your butcher butcher a tame animal if you have one.
If you cannot satisfy the mood, there is no way to avoid losing the dwarf. He may go berserk and put others at risk- wall off the workshop to prevent him harming others. You might as well prepare a coffin for him ahead of time- no sense leaving his body around too long.
There is no cure for insane dwarfs. They will never recover and will eventually die, either of starvation, suicide, or being killed by other dwarfs