Temporary blindness in my cat our vets can't understand

I have an almost 13 year old cat, he's no particular breed as he was a free accidental kitten we got from a family friend, he's got long hair, he's grey and white, and he has a faint tabby 'M' on his forehead. His brother was a black short hair and his sister was a calico. Vets often ask if he's part maincoon as hes huge and funny looking.

He's my cat but i don't live with my family full time as I go to uni in a different city so I only get to see him when I visit, as of January 1st I noticed he has temporary spouts of blindness followed by perfect vision the next day. We've taken him to the vets twice for this and he's had his blood looked at and his blood pressure measured.

We've spent £200 to be told our cat is exceptionally healthy for his age and they don't know what's causing it, we were given the option to take him to an eye specialist but they're quite a drive away and a consultation alone costs £300. He gets stressed about travelling and I don't want to make my parents fork out more money if he's not actually suffering and if there's no guarantee they can tell us what's wrong (he's a bit freaked out when he can't see but he wanders around fine and we've had a cat that went permanently blind from old age before so we know we can help him manage if that happens).

His main symptoms are wide cloudy pupils, the green of his eyes goes reddish and it only really happens when it's a bit darker, I don't think it's ever happened during the day. The vet told us it's unlikely a brain tumour or glaucoma as if it were a tumour he'd be permanently blind and if it was glaucoma his blood pressure would be funny

I just wanna make sure I'm doing the right thing by him as he is old and seems perfectly healthy according to his blood, he's still pretty active (he's quit hunting thank god, but he still loves watching the birds in the garden and playing with my shoelaces and earphone wires).

If anyone has any suggestions about what this could be or what I could do to help him I'd really appreciate it!