Should I just let it go?

My son lent his bike to a friend a year ago. I've asked this boy to return the bike repeatedly and he says "Oh, yes! Tomorrow", but never does. Today, I ran into him with his mom and I ask about the bike (I'd asked him as recently as yesterday) and he said "Oh, I just found out yesterday it was stolen. It was stolen once before, but I found it and brought it home."
His mom just stood there and offered no apology or any kind of support for me. They left quickly. Should I just let this go? Remember, he's refused to return the bike for a year and now, he says it's stolen. I don't want to cause any issues, but I am burning mad. I think it's because of his irresponsibility and the fact that I think he's been playing me all along and the way his mother acted. What should I do?