Buying a house with current tenant - seller's family

[Nick Michaels]

So I have an accepted offer on a duplex where the bottom unit is currently occupied by the seller's son, who wasn't paying any rent at the time - I knew this before I made the offer.

I spoke to him (the son) briefly yesterday as I was at the property for the home inspection - he said that him living there has been part of his "compensation package" from his father for managing his father's properties. Obviously that is going to change. He also seems like a pretty reasonable guy and keeps a tidy apartment. I can see it from his perspective of going from paying $0 in rent to paying $X is rough, but I certainly don't want to get into a negotiation after closing with the son.

My question is - I got the inspection back and plan on making some requests anyways -  should I / can I require that I have a signed lease from the son before closing? 

Thanks!