If you have any doubts whatsoever about him wanting the money back then don't do it. Not worth risking family relationships over.
If you do decide to do then it's really worth getting some legal documents drawn up and signed by everyone with witnesses to make sure it all goes as planned.
My dad lent me the money for my car (�5,000) and I paid him back monthly without interest (I know, I know - I'm very lucky!). We drew up a document together and got two neighbours to witness and sign it. I don't know how legally binding this would have been but it gave us peace of mind (not that I'd ever rip my dad off!)