As he is struggling even with treat food definately take him asap, tooth ache is one of the worse aches!
Abit of info you may find interesting, although cats are physically wellcovered without food they are very quickly at risk of liver problems, overweight cats are at an even higher risk, hepatic lipidosis is its posh name but is more often referred to as fatty liver, the body sends fat to be processed but cats livers cant do that very well so the fat builds up