When you loan out a substantial amt of $$ to a friend...?

My boyfriend loaned a substantial amt to a friend who agreed to pay it back in installments. These were usually cash loans or ComCheck loans. We don't have any real way to prove that he owes the $ other than the two talking. Now the friend hasn't paid on the loan and is refusing to make any payemnts. What types of recourse do we have to recoup the money loaned if any??