What should my boyfriend do about his damaged ed visa stamp before our flight back to the UK?

@Sa***

if he's down to 3 pages that would change things, he's going to have to get a new one soon anyway so yes may as well do it now.

Definitely replace it if there are only a few pages left and bring the cancelled passport with you for transfer to the new passport. I had a horrible experience recently with a wet passport on a few pages that I thought would be 'no big deal' but it was a huge problem.

He has the reentry permit already. That stamp isn't going to matter beyond his return, they do not go back through every page in your passport judging old stamps for water damage. There is a point at which water damage can be an issue requiring a new passport but that really isn't that bad IMO, I have had a passport worse than that on the visa pages that I continued to use until it was done without issue. That is very slight water damage that could constitute "normal wear and tear".

If the photo page is 100% intact I would not see the need to replace that. Up to you, but a new passport is not free and is not zero hassle either, and you'd also then have to carry both to get back in and then get the stamp transferred.

Only in Thailand. Extension of Stay and re-entry stamps are from Immigration, not used by Thai Consulates.

yest he can, extensions of stays CAN be transferred to a new passport at the immigration office