Add me to the list of those who are curious why Doug was invited if DH suspects his anxiety issues might make the trip unpleasant. If DH was aware of them before issuing the invitation, I don't think he can rescind it now. I like the idea of asking Doug to "confirm" if he's really coming, maybe by email--if Doug felt pressured to accept (maybe it was an on-the-spot thing), this could be his out if he's looking for one.
If Doug still wants to go, it might be good for DH to talk to him about the trip and what DH plans to do with his own time, so Doug can make his own arrangements. Then DH can be clear about his expectations--for example, that they will spend some time together and some time apart. I don't think DH needs to be glued to Doug 24/7, but Doug is his guest, and he should at least provide Doug with some options--like someone else said, "Doug, I'll be going surfing Tuesday morning, but if you'd rather not join me, there's a historical tour you might like to go on that leaves the hotel at 9am." I assume they would be talking extensively anyway, like about what the award is paying for and what DH will be paying for and what Doug will be responsible for himself, money-wise. This advice would apply no matter what, even if Doug was a great traveler; but it will also give him the chance to figure out problem spots in advance and solutions for them.