[Suicide Junkie]

Well, you could hit him with some missiles (from outside of his weapons range, hopefully) in the short term, until he redesigns his fleets with more point defense... you should be able to kill off a decent part of his current fleet, and future ships won't be quite so deadly against your direct fire warships (less guns, more PD).

For the most part, Stock does not have many viable weapons, and if you fall behind in researching them you get burned hard.