People at pet stores generally are not very knowledgeable and I would not take any medical advice from them. (Apologies to any knowledgeable pet store employees here!
)
Dogs seem to LOVE string cheese, but most cold cuts have too many additives and salt and I would not give them.
Another option is baby food - chicken and rice, turkey, etc. Few dogs can resist that. You may have to open his mouth and put some on the roof of his mouth to get him started.
No-salt chicken broth or beef broth added to his food may help. Hope he's feeling better soon.