Does he go out and have access to grass? Cats use grass to help them throw up hairballs. Sometimes this results in nothing but frothy slime with a bit of grass in it.
There is a marked difference between the way cats throw up when they have hairballs to when they are ill. With hair (or fur) balls, they cough in a deliberate way, as though they are really trying. When they are ill, they are overtaken by the rush of vomit. (Sorry to be so graphic
)
A few things to check: your cat's ears should be cool, unless he has been sitting in a particularly warm place. If his ears are hot, this may indicate a raised temperature.
Has your cat been vaccinated against feline enteritis? This usually produces a nasty yellow vomit and diarrohea.
Otherwise, get one of the many furball treatments available: katalax, defurrum etc. Don't buy it from a vet, they charge a fortune!!
If your cat likes fish (he must like kippers, could he have had a bone stuck? Just a passing thought!) give him some tinned tuna IN OIL NOT BRINE as this helps to pass the fur.