The son probably owns both properties now, yes? Have the son sell the house that you want the dad to live in. Then move the dad into that house and he'll live his life like any NPC.
With additional properties, one is always designated as "home" for the entire family. Can't change that. Mail and newspaper delivery are effected by this. However, you still have access to the properties just like you lived there. Just keep in mind that if you tell your sim to "Go Home", they will go to the primary property.
Now if you wanted, for example, to still play the dad in the household, you could still do that and have him 'live' at the non-primary property. Just don't tell him to "Go Home" and use the Set Bed Ownership on his bed at the old house. You could also use the new locking functionality on doors to exclude the dad from access to the outer doors on the primary property to give the son and his family their privacy.
Some people are coming up with cool ideas on how to use the additional properties. I saw someone say they were going to try buying an additional property and turn on the roommates on that property to fill it and then, basically, collect rent as a money making scheme. I'll be curious to see how that turns out.
For myself, I love doing a horse ranch but have always been a bit limited on how big I could go because of space. Not anymore. I can have multiple large properties for spacious grazing, a separate property for training facilities and I'm considering a separate property for pregnant and nursing mares. I'm still in the planning stages in my head and looking for the right town to do it in.
Let us know how it turns out with the dad.
