I developed breast enlargement after RT and while on HT - Prostap in my case - and my nipples became very tender. Even clothing rubbing against them made them sore. Mentioned this to consultant who offered Tamoxifen but I am not good with pill type medication. She agreed to give a RT zap to each breast bud and in reality it made little difference. This was 15 months after RT had finished (I can remember this as I had recently moved house).
I'm on a hormone holiday (intermittent treatment) and the breast enlargement has receded back to normal with no nipple pain or tenderness. That took around 6-9 months after stopping hormones.
Agree with the above, he should not stop his treatment but maybe ask if he can have intermittent hormone treatment like myself. If you are worried about the outcome, don't be, the results for intermittent treatment effectiveness are the same as permanent treatment. Or, maybe ask if he can try a different HT brand.