The vets did all the nec tests and said they couldnt find anything wrong (and his temperature was good). I then mentioned his runny poops and they said this was a concern because cats shouldnt have it as often as he does - once a month is fine but anything more then that should be a cause for concern and he has it a few times a week

After leaving the vets, he was put on a prescribed diet for four days eating Hils Prescription pouches. His poops were solid so I thought he was on the mend but today I fed him what Ive been feeding him since I rehomed him (in Sept): Bozita Chicken and turkey and yesterday I fed him Animonda Carny for the first time and again he had diarrhoea (and today it was explosive).
He is acting normal and has a good appetite and this gourmet food is high in meat content so Im not sure whats going on? He doesn't drink much water but he mainly eats wet food which is high in moisture so I feel a bit lost. Dont really want to have to keep going back to the vets and paying £30 when its not a very very serious matter
Edit: He is an indoor cat