And it has ruined several pairs of shoes including two pairs of suede Merrells
My god, it sounds like the man did you a favour. Give him a break!
Anyway, if he held himself out to be qualified to do the job properly, then he would be required to do it with the care expected of someone with suitable experience and knowledge. That would also likely include checking that all valves are properly switched off and that there are no leaks. He didn’t make this check, and so is responsible for the likely consequences (i.e. water damage).
So, in short he – or, if he’s sensible, his insurers – are on the hook to put it right. However, without getting too difficult, you need to strike a deal as to what level (£) he is going to agree to cover, taking into account the nature and age of the items damaged.
The danger with his insurance company becoming involved is that they’ll be even harder to neotiate with, and will fight even more not to pay up and blame you. So, best to deal with him.