He may also have 1 or 2 decaying Back Teeth which the Vet will need to look at and remove if necesary. Happened to one of my cats in the past but they manage OK even if they have teeth extracted.
Take him to Vet as 237SJ suggests.
this also happene to our cat [Violet Elizabeth Bott] though only 5 at the time she to have 4 teeth removed,she is fine now but we give her senior biscuits and jelly pouches because of the missing teeth.
Oooh I hadn't thought of rotting teeth - I thought some breath fresh chews, like dentistix (sp?) for dogs, but can't seem to find anything. Looks like a trip to the vet is in order, before he gets a complex about us singing 'smelly cat' every time he walks past! lol!
Yep, I would confirm the possible cause as teeth - tartar buildup etc. is common in cats too. My vet peers at my cats' teeth every time they go. Tartar can be prevented by eating enough biscuit but occasionally they have to go in for a tooth-clean under light sedation. Mind you, their breath smells pretty honky on a good day anyway, sort of fishy-meaty-biscuity in your face!
lol, yeah his breath is normally pretty bad, maybe I'm just more sensitive to it because I'm pregnant - I think I'll get someone else to sniff him before I spend 30+ quid taking him to vet!