Dog vomiting yellow foam

[vx220]

Hi, our Westie has awoke early in the morning and vomited yellow foam a few times since we've had him (he's nearly 8, had him since he was six) but has just done it three mornings on the trot.

When we got him, he was well fed, but a total scavenger (still is) so we feed him four small meals a day. In between meals, he is licking his bedding a lot. Last night, having been told dogs vomit like that if they're hungry, we gave him an extra few bites of his normal food (Wainwrights) but he did it again this morning.

Any ideas? He's absolutely fine in himself, no recent issues ingesting something (afaik) he's walked on the lead because he has no recall but has on occasion picked stuff up and eaten it (once a dead bird, got most of it out, no ill effects). He doesn't feel bloated or thin, isn't sensitive around his belly.

Thank you in advance