Thanks to all those who answered. I took my son on holiday and we had a great time but were effectively uninsured. Literally no one would insure us with pre-existing conditions and less than 6 months on from the end of radiotherapy. I went to Insure With, Good to Go, All Clear, Freedom Travel, Holiday Extras etc. All said no.
For us, I felt this risk was worth it as it was extrememly unlikely anything medical would go wrong and, if it did, I know all the hospitals in the country we went to are excellent and actually have better brain tumour treatment programmes than the UK (so I knew we'd be in good hands should an emergency occur). We also both have EU passports so, again, it makes the potential cost of overseas medical expenses less worrying.
If my son's health were less stable, or I was travelling to a place I'd never been and wasn't certain about local hospitals, I would have been more concerned about obtaining proper insurance. As it was, I was only worried the airline would lose out luggage, so I'd be out a few hundred pounds for new clothes and a suitcase.
One thing I did do, which I would suggest anyone in a similar position look into, is separate wheelchair insurance, if your child is using one. My son has one that was provided by the NHS and a condition of taking the chair on holiday is you get your own insurance for it. I used Blue Badge Mobility Insurance - I didn't spend a great deal of time looking around at whether they were competitive or offered the best options; I just needed to get something in place quickly. In the end, I didn't need to make a claim, as the airline didn't damage or lose the wheelchair; so I can't speak as to how helpful their claims process is. (Take the chair seat cushion and footrests off and put them in your cabin baggage. Apparently these are the things that, more often than not, get lost or damaged - the ground crew confirmed this when they commented approvingly that I was already taking them off when they arrived.)