In Maryland, you'd file a suit against your nephew for the full amount at the local District Court. Having a document, signed by both parties, that attests to the loan would help greatly, however, if you don't have documentation memoralizing the loan, an argument may be made that your nephew was unjustly enriched by your loan. This line of argument is very complicated, consequently, I'd recommend you contact an attorney to determine whether this is a suitable option in your state.
It will be harder to prove without a written document, but you can sue him is small claims court for breach of contract. The real issue will be whether your nephew has the money to pay your or has property that can be legally attached and sold to recoup your loss.
You may not need an attorney. Hopefully, you have something in writing to document the loan. If so, you can file a small claims court suit to recover against your nephew. Small claims court is the cheapest and fastest way to secure a judgment for your money.
Collecting it may be another story if your nephew has no non-exempt assets. Check out my legal guide on the Texas Property Code and what assets are exempt from seizure by creditors. But, who knows, maybe your nephew will play and win the Texas lottery.