I would refuse to discuss it until closer to the date.
Right now, people have different levels of fear.
The papers are full of stories that we won't have any more cases in July, now, that could be bullshit, or it could be right. If it is right, if by July this is pretty much gone, he might well relax at that point.
If you don't need to cancel until September, hold off until the summer to make the decision.
I can see if you are a planner you want something to look forward to, but try to relax, tell yourself if you can't go to Turkey, you'll cancel and use the money to book a last minute deal in the summer.
Hold off having the argument because it's most likely to get your DH to entrench his views. take a deep breathe, tell him as you don't need to cancel until September, let's not think about it until August, at least that way you'll have a better idea how things are going and what sort of risks you'll be facing.
I know it's tough if you are a planner to not plan!