What legal action can I take in this matter?

I bought a house back in 2005, but due to some problems I had to leave the States and couldn't continue to make the payments. Before I left I had a signed and notarized agreement with a co-worker that he would continue to make the payments to the house and would go under his name when done paying it. Now I can go back to the States. And just found out that he stopped making the payments since Jan. 2009 but he is still living there, he owes more than $8000 and I want him to pay that money because he still resides at "my house", I don't want to be an *ss about it but its still my house since the loan is still under my name. I don't want the house to be foreclosed and lose a great deal on that property. If he pays it the deal still goes on, the only thing is that I don't want to damage my credit more than it is right now.