Your husband's diagnosis is almost identical to mine, but mine was T3 (I presume T3a) not T3b. I was not offered chemo, and at time (7 years ago) chemo would not be offered to anyone less than T4, however HT was prolonged over two to three years. So maybe chemo is being offered so they can reduce HT time.
The side effects of chemo for prostate cancer usually docataxol are usually well tolerated, but many would consider it an aggressive treatment if it is not really required.
The following figures I am quoting are the approximate cure rates. If you take 100 men with your husbands diagnosis and treat them with RT (and HT) 69 will be cured and 31 won't be cured. If you give all 100 men chemo as well, then 71 will be cured and 29 won't be. That means by giving chemo to everyone 98 will have had chemo for no benefit, it didn't change the outcome, however 2 out of 100 switched from not cured to cured. It is impossible to know in advance (or in hindsight) whether you are one of the 2 people it will help or the 98 it won't help. Your husband needs to decide if chemo is worth a 2% chance of improving his outcome. (All these figures above are approximate values, your oncologist may be able to give you more accurate values).
A female friend of mine had breast cancer, it was treated early and she was offered chemo as well, 'just in case'. She declined, ten years later she is perfectly fine.