Adrian Flux have been very competitive for me the past couple of years - the '5 is modified and they were the cheapest quote and offered the best cover too! Not sure about classics, but I've heard good things of Footman James.
Can't you add him on your existing policy as an additional driver? He's really going to struggle to get seperate insurance for it when a) he only needs it 'for a bit' b) the car isn't his and c) its already insured (on your policy).