I have an English Mastiff, and she suddenly stopped eating one day. I took her to the vet, and it turned out she had a kidney-stone.
I'm not saying that's what yours has, but pain (according to the vet) will cause a sudden decrease in appetite.
Is your Mastiff urinating well? Meaning, for the amount that he's intaking, does it look like he's outputting enough?
Hello & welcome.....From your limited information, it would be impossible for us to suggest what's going on with your dog.....
His drool being more than usual suggests Nausea or a Bad tooth.....Both would create loss of appetite......My suggestion would be to get him to a Vet ASAP.....He needs to be checked out before it gets worse....With the weekend coming, you don't want this to turn into an Emergency room visit.....It would be less expensive to take care of him now, at your regular Vet's office...
Good luck and please come back to let us know what you find out.......Karla
P.S. If you feed him Hot dogs on a regular basis, this could be a case of Pancreatitis which is not only very painful, but dangerous if left untreated......Make the call, now!