Bellasnana, I think the mouthwash was what Galen mentioned, Corsodyl, and also an anti-fungal suspension for the mouth called Nystatin. It is an unfortunate side-effect and mouth ulcers will make him feel more miserable just when he needs good nourishing food, but good soups (supplemented with protein powder as Galen suggested), good purees, custards, bananas and avocados (a good thing that he likes them) will all help him to keep his strength up.
'Cheers' to you, soop and MacS (or 'Cheese' as DGD1 used to say as she lifted her sipper cup of water to clash with our wine glasses) and hope a wee dram cheers him up.
A bit chillier here for the Bank Holiday weekend but the sun keeps peeping out from behind the clouds trying to make an appearance.
I hope everyone has a good weekend despite their problems; relaxing (or doing whatever you most enjoy).