You'll get a lot of replies to that question, Naboocat. Many of us on here, myself included, favour a predominantly wet diet in order to ensure adequate fluid intake. There are a number of high quality wet foods on the market - that is to say, foods with a high meat content. But in this case, it's first finding something that Naboo will eat that settles his tummy a bit. There are several people on here who will be able to advise on a bland diet for him in the short term.
I am not a vet and my response must be read in that context but if Naboo has been checked by the rescue's vet and found to be in reasonable health, it could be that his digestive system is still adjusting to a proper diet instead of the scraps and so on that he will have had to eat as a stray. Nonetheless, I would be inclined to take him to my own vet, if I were you, to be on the safe side. An overall check-up will do no harm, anyway.
