Lead is the only one I can think of that might be on the watch an it is pretty harmless touching your skin. When you get in your lungs an mouth though that it can poison you. Mind you as a mechanic, he is handling lead daily along with other bad things. Lead does not really rust like some metals, so it is probably a steal watch coated in tin. (pretty harmless)
Remember fishermen an gun people handle pure lead all the time too. The danger comes from kids putting it in their mouth.
If you want you can keep a coat of clear coat nail polish on the back of his watches to keep the metal off his skin.