My husband also had a bone scan, I really think it was just to get a "starting point" because the ADTs had cause osteoporosis later on..... in our case, the survival rate is low anyways, so I really don't think my husband has to worry to much about osteoporosis, THOUGH the cancer had spread to his bones, I didn't see the scan, just the write-up and it's all over his body, it was a fight to get him to start ADT, but the pharmacist pretty much yelled at him saying to start treatment right away or he might have a break or worse a spinal or pelvic break that could land him in a wheelchair etc. It's all very scary!
My husband is "young" and fit, he goes to the gym daily, so it's different than your dad, but if he has brittle bones now and not steady, there's a high chance of a break?!
My in-laws got Prolia shots for maybe 10yrs or more, I have no idea if your dad would qualify for that or not, I know they couldn't have surgery or teeth extracted while on Prolia, or at least not for several months maybe?
I bet you don't want to encourage a wheelchair, my mom was using a wheelchair the last few years of her life and she got worse and worse, so....