I can't offer personal experience, only that of both my parents, one of whom has had RT and the other of whom had the colostomy bag.
Mum - colostomy was following emergency surgery for peritonitis due to burst diverticulum, so very different from the planned surgery your Dad will have. However, outcome pretty much the same, as although Mum technically could have had a reversal some time later, she chose not to - your Dad won't have that choice.
Living with the bag took some getting used to - things that Mum had done were to move into a separate bedroom (luckily our house had the space) and to have a shower and sink unit installed in there, so that she could clean herself up in private (en suite bathroom not an option).
Parents house also had a downstairs loo, with a small sink; a flexible shower hose and water heater were installed in there as well.
I don't know if things have changed now (probably) but a good stoma nurse is really helpful - one who can help your Dad get used to doing his bags by himself. Stoma bags come with different shape "holes" - some are round, some are oval, and some will fit better than others. The supplies used to be free, including the disposal bags (like nappy bin liners) and dry wipes as well as the stoma bags themselves. After a while, Mum just got used to dealing with it all, but she was very private about it and hated being out for too long.
Also, diet-wise, he will have to be careful about the sort of veg he eats - baked beans are out for starters! Anything likely to produce large amounts of wind (legumes are a nightmare for this) are not to be recommended, as the wind build up can break the seal on the stoma bag.
Making sure the skin is clean and dry before applying the stoma bag reduces the risk of "leakage" - something Mum was utterly paranoid about, especially the smell - hence all the cleaning stuff everywhere.
If there is any residual large bowel left after his op, it might "leak" a bit for some time afterwards - this is just the mucus that the membranes produce to help lubricate the anus for stool evacuation. But it can go on for quite a while!
Dad - had RT for prostate cancer. Very focused, but he still got a bit of "sunburn" (radiation burn), which he found aloe gel helped with. What got him was the fact that he started off feeling fine but after about the 4th treatment, he started to get very tired, really seriously fatigued. And it continued until he finished. At first he would try to work through the tiredness, but luckily he had a "cancer support group" and they told him to just give in to the tiredness and REST. So he did, because really, he had to!
Hope that helps, even though it's two different people.