Don't actually give it to him unless it's too big to be swallowed. Minting a real coin, as those last couple of companies linked in the posts do, isn't likely to be very affordable, unless you want to make a whole lot of them. 3D printing directly in metal isn't going to be cheap either. The most economical path to this is to have a designer make up the 3D model (or do that yourself), have it printed in a high-resolution castable resin or wax, then send it to a casting service which is willing to do one-offs. Shapeways has a lot of designers on their site, and works with casting services as well.