Hello again Emma.
We have never had trouble getting insurance for John's PC, even treated.
When he was on AS he declared it and at most it put the premium up by a few pounds.
The thing about holiday insurance is that you are unlikely to need cover for PC whilst you are away.
There is nothing going to happen in a fortnight away that will require Mart's PC to be an emergency issue.
I now do comparisons websites for our holiday insurance. I do declare everything, including PC, and let the comparison website sort out which insurers will cover a man with cancer.
Some on here get cover via their banks. Some might not even declare the PC if they are at your stage since you probably won't be rushed off to hospital with PC symptoms if he is at the AS stage but I'm not suggesting you do that.
Check your bank doesn't offer you holiday insurance. IF they do, contact them and explain and you may be pleasantly surprised to find Mart's PC is not currently an insurance issue