Cranberrry juice may help. I was drinking it all through RT (although I don't know if it helped). If you are on a low fibre diet, make sure it's a clear cranberry juice. I used the Ocean Spray Light cartons (otherwise you will be consuming loads of sugar too).
RT hits the bottom of the bladder and irritates it, and thus reduces bladder capacity. Added to this, it hits the internal urinary sphincter and weakens that, which means you are likely to become at least a little incontinent during RT. Get some mens incontinence pants in preparation, and I suggest wearing them on the treatment table. Also, he is doing pelvic floor exercises I hope? This will help work around incontinence in some cases, strengthening the external urinary sphincter to use when the internal one leaks.
The low flow rate is because the prostate becomes inflamed. This is expected too. These symptoms usually all start clearing up 2 weeks after RT finishes.