First, I would see how much gain he is using on the mic preamp on the desks channel.
If it is still too 'hot', then I would first try lowering your input gain on the amp and upping your master volume to keep the level, but send a lower signal to the desk (?)
If that doesn't work, get a seperate DI box (Behringer make one for about £20 - I know it's Behringer, but their DI's do the job nicely - we use a few of them or our guitars). It should have a 20Db cut switch on the DI, so that you can send a lower signal to the desk. Also, it will mean you have a good backup in your kit if the amp ever dies. This will mean connecting your amp to the DI box with a jack to jack using the amps line out and then use an XRL to connect the DI to the desk.
Hope this makes sense.