Aggressive doesn't always mean what you think it means, particularly in relation to prostate cancer. Aggressiveness is determined by how distorted the cancer cells are; your dad will have been given a Gleason score which will look something like G8(4+4) or G9(4+5) or similar.
Not all hospitals do bone scans for all men; they use the information available to them to decide whether there is a risk or likelihood of it having spread. The fact that they have offered him surgery indicates that they believe the cancer is still contained in the prostate, otherwise they wouldn't be doing it. Their risk assessment will be based on the PSA, the Gleason score, how many samples were positive, how much cancer there was in each of the positive samples and where in each sample it was positioned (near the middle, well inside or near the edge). Has he had an MRI scan and just not mentioned it to you with everything else going on? It may have been before his biopsy?
I am not sure why you are concerned about the 3 nights in hospital? It is major surgery so the hospital will want to know that he is able to move around easily before they let him go home, particularly if they know he is a carer for your mum. Sometimes the hospital says 3 nights but it is only actually 2 nights if all goes well and they need the bed. Personally, I would want him in hospital for as long as possible as the pain relief is better.
There are obviously diagnostic results that you either don't know or just haven't told us, that would make sense of your dad's situation. It would be a great shame if he has been rushed into agreeing the operation without an opportunity to talk to an oncologist to consider alternatives such as radiotherapy or just monitoring it for a while, but I guess he is unlikely to want to delay the operation just because of something you have read on here. The best thing you could do for yourself and your parents is phone the number at the top of this page and order the toolkit - that will explain the different types of prostate cancer, what the results mean, and info about surgery and other options.