Physical. Anything metalic causes him to shunt power, so he has to stay insulated himself. He could also be sustained by electicity. He needs it like food, or maybe even air as he becomes more powerful. Hitting him with anything metallic could cause him to discharge too much energy. The PCs could also track him down by finding orders for genetators, high electric bills, etc.
He could draw the energy from an area, either inadvertantly, or out of necesity. Being insulated from the outside, RF heavy world could diminish his power. This could also mean the PCs take damage when near him as he drws the energy right from their bodies. This could be damage or a cumalative save vs fatuige, exhaustion, unconciousness. If he could be trapped in a non cunductive RF shielded material, he could be captured. Drawing too much energy could overcharge him.
If he has the ability to tap into machinery or power grids, he could be vulnerable to anything that would affect these things. Power shutting off, a power surge during a storm, power being drawn into a battery, etc.
Mentally, lets say he got his powers from being struck by lightning, (classic/cliche). This could cause him to be absolutly phobic about rain or lightning storms. At least those which he dosn't control himself, if those are the powers you envision him having.
With an above suggestion, quartz bullets sound kinda cool. They could do extra damage and pass thru any protective power he has in place. Quartz bolas and silicon ropes could ground him out, causing an casting failure chance, ala D&D.
