Just a thought, I know when I get UTIs, (very prone but no problems structurally, I just have a urtethra on the narrow end of normal so more prone) I HAVE to have probiotics afterwards because if I dont I get recurring UTIs from the lack of good bacteria helping to stave off any bad bacteria. They are all genuine infections, but it wouldn't necessarily have happened if I hadve taken probiotics iykwim?
Could you try giving him some and seeing if that helps? He can even take them during the course of ABs, just take the probiotics at least a few hrs after the ABs.
I only suggest that because he's never had one before this so it seems strange to get so many all of a sudden at his age.

Only other thing I can think of is do you still help shower/bathe him? If not, maybe a possibility could be that he may not be cleaning himself properly?