If you want to insure him, then take this minor episode in your stride. Gastro illness is not usually something that would require insurance to cover. Usually injuries related to accidents, bladder infections and later life onset illnesses are what cost the money, and these issues would still be covered.
Your vet will have noted his record that the episode was a one-off and therefore not a chronic condition.
If I were you I'd shop around for the policy you want and tell the insurers he's had a bout of "kitten tummy", I'm sure it won't affect the premium too much.
I've insured my accident prone "outside" boys and they cost me £35 per month due to their rather chequered history, this means I cannot afford to insure the other 5, but we have a fund for their vet treatment which does cover their costs. If a big bill comes in for one of them we'll cope with it when it happens.
I wish I'd insured Sly when he was young, though, but didn't because a vet diagnosed a minor heart murmur. Since then he's developed a neurological disease which affects his legs and the prognosis is not too good, so despite him only being 4 years old and otherwise healthy, his medical treatment is going to get expensive as the diagnostics can only be achieved by MRI scan. This would have been covered despite the early diagnosis of heart murmur so we stand to be seriously out of pocket when his wobbly legs finally become too weak and surgery is needed.
