Yellow foamy stuff is probably bile. His tummy is probably empty. When is the last time he ate? Did you maybe feed him his last meal earlier than usual? Or is he due for a meal but he is getting it later than usual? I know some dogs if there tummy is empty they may get a little upset stomach from it.

If he's acting fine I'd probably feed him a little bit of food. Just enough to put some in his belly. Wait an hour or so & see how he is. If he's still fine you can feed him the rest of that meal.