I would caution you some on probiotics. We're in the Civil War understanding of the gut microbiome, what bacterium are helpful and hurtful. What we do today with probiotics is probably no better than putting leeches on someone to cure their depression (or amputating a limb rather than treating the injury as we would with modern medicine).
While there are tests like the uBiome which analyze your bacterium from stool, the results back are very broad and vague. They only tell you the family of the bacterium, not the species. Most aren't as familiar with bacterium families, so let's start with something more familiar. As an example, Hominidae is the family of humans and apes. So if your report told you Hominidae, then you do not know if you are talking a chimp or a human. So, you cannot custom order the bacterium you need to be well, because the classification of that report is so broad and non-specific.
Probiotics did nothing for me, so there's no guarantee they'll help your son. I do sense a lot of health anxiety and worry from you, which is totally understandable. Do be a little cautious though, and know there are no easy answers to this whole UC thing. Know it is going to take a lot of time, patience, and a bit of trust within his doctor.