There is very little control you have over this. 

His parents are in their 90’s and still live in their home but he is wanting to buy their home to take one thing off their plate.

You get to control that emotional message.

Parents can give up to $15K per year for children.

Some examples include family's with adult children and/or an aging parent that needs to be close to family/care.Lastly, I wouldn't necessarily agree that if something was possible a prior owner would have done it already.

I don't know where you live, but, this sounds like something you would have to consider in California or New York where they have Rent Control, not in Florida.

Use their crew with total control of the work flow and the speed at which it gets done.   

The issue is, I am on the deed to my parents' primary residence, but not on the mortgage.

In my mind the bigger risk is of new parents deciding not to return to work after their leave runs out. 

He is current on rent so far and no issues but, I was asked if my boyfriend parents can move in with him.