Someone here on YT recommended this:
Multi Strained Probiotics | Cure for Ulcerative Colitis
You can get the powder form & give 1/16th of a teaspoon. It needs to be refrigerated...hope you'll be able to find some in the UK. I ordered it from amazon, it's expensive ($86+ for 30 sachets). Any good probiotic will do, yogurt alone will not help, he'll need to consume alot of it to do anything. He needs to be on a high fiber low fat diet. Add pumpkin, green beans, etc & avoid red meat!