trojen,
I am so sorry your son has to deal with UC, let along a recurrence of infections.
I would suggest you look into:
1) has his blood levels of vitamin b12 and d been checked?
2) is he on probiotics? VSL#3 DS ( via prescription) are the best ones with 900 billion active cultures
3) LDN - low dose naltrexone, not only does it put people with UC and Crohns in remission, but it generally boosts the immune system, which it sounds like he really could use.
What do you think?