IF (and this is a big if) all possible physical problems have been ruled out (and I would definitely include teeth check), then I would:
1)exercise the dog more (long on leash walks are best), it will build hunger
2)make sure clean, fresh water is constantly available and is being consumed
3)make sure food is being given at a quiet time in a quiet environment (we have one dog who will only eat in his crate, where he feels "safe")
4)put food down with minimum of fuss at regularly scheduled times, give verbal cue (I sing the "Suppertime" song from the Charlie Brown musical, but you might now want to go that far
), leave dog alone with food for X amount of minutes, pick up food at end of time, and do not offer again until the next regularly scheduled feeding time (I do 20 minutes, but it's up to you)
5)give absolutely no other food/treats/people scraps/etc
If you had a Whippet, I would worry, but I've never seen a skinny Pug
.
Good luck!
Terri