Cats naturally look for somewhere with loose soil to pee and poo. A litter tray should be his natural first choice anyway as long as you position it where he won't feel threatened. (A cat isn't to stop for a poo if he feels that there's a predator nearby. To start with he might see you as a threat, so you need to ensure that his litter tray isn't in an area where you're constantly busy; put it somewhere quiet).
Cat's normally regulate their own feeding. I've got four of the little monsters - er, I mean 'darlings' - and I simply provide a running buffet for them. (i.e. if their food bowls are empty I fill them up straight way; they only ever eat as much as they need).
If your new friend's tummy doesn't settle down, forget about the advice from the RSPCA and try some different food. (When the mother of my foursome was nursing them, the vet told me that she must have pouched kitten food herself but she simply couldn't stop producing liquid poo. Then a lady who'd had loads of nursing mother cats to deal with told me to forget that and simply give her normal cat food instead. Her advice turned out to be far better than that from the vet!).
My lot get a mixture of Lidl's 'premium' dried food ('Coshida Selection' which comes in plastic bags, rather than the ordinary 'Coshida' in boxes) and the pouches that Lidl also sell.
Enjoy your new friend's company. Welcome to a life of slavery!
;-)