Kill Him
Kill him, Kill him, Kill him. But Roleplay it out. A little. He's a prisoner, bound and gagged and caged (because they know he's a druid and what he can do). The orc chieftan talks to him for a few minutes, then executes him.
Of course, that's not the end of all things for most PCs, so you can have the party find his body and raise him from the dead.
I had a similar situation in my campaign. The party's elven archer surrendered to overwhemling odds when he was cornered away from the rest of the party, and was dragged before the big bad dragon that was at the heart of the adenture (the adventure as designed so the party would never actually meet the dragon, but things get screwed up every once in a while).
Anyway, in some cases I might have had the dragon dominate or charm the PC and turn him into a puppet, but elves are immune to enchantment, so that wasn't really an option. I was left with very few options that made any sense -- the most sensible thing to do was to kill him. So the dragon impaled him on a long claw and gave the body to it's tribe of Troglodyte servants to eat.
Of course, that left the party in the middle of a dungeon and the archer's player with nothing to play. I had set up the adventure with several lizard-humanoid types serving the dragon - Kobolds, Lizardfolk, and trogs, and the Lizardfolk were potential allies for the party, as they were not entirely happy with their service to the dragon.
I also needed to give the party some urgency in their efforts to retrieve the elf's body and get out of the dungeon. The rest of the party was already hiding in the Lizardfolk enclave, and one of the lizardfolk lieutenants had been present when the elf was killed, and was able to bring back a report to them of what had happened.
So, for urgency, the Lizardfolk told the party that Trogs, much like crocodiles, like their food a bit rotted. They would have taken the elf's body back to their camp and stuck him underwater, where he would soak and rot for a few days before they actually settled down to eat him. That gave the party a timeframe to work with, and some real hope that they wouldn't be taking back bags of Trog droppings to have clerics try to raise from the dead.
Then, to give the Archer a character to play, I had the Lizardman Lieutenant agree to act as a guide for the party -- in exchange for certain guarantees for the lizardfolk. The party managed to rescue the elf's waterlogged, rotted body, although the lizardman died in the attempt, and they were eventually able to bring the Elf back.
Anyway, now that my self-serving story is over, I'd say you might look for something along the same lines for the Druid's player to do while the party tries to recover the body and bring him back. There might be all sorts of possibilities, creatures that the party might meet out there that might make interesting temporary allies, with their own agendas, that the Druid's character could play while he's waiting for his character to we revived.