My questions are, do we need to feed him the same food he ate at the rescue? Depends if he's fussy or not. You don't have to. We switched our rescue to a better food when we got her.
Do we leave dry food and water out all day? Water yes. With food you need to figure out how much he needs per day, based on his weight, and then choose how to feed him from that allocation (wet, dry, snacks, etc). If you overfeed him he'll gain weight. Some cats eat all their food as soon as you put it out, others leave some for later.
What do we do if he scratches or hisses? Some cats are arseholes and will scratch, bite and hiss, but most do it when they are frightened, feel threatened, defensive, etc and are just trying to get away from you. If this happens, just back off and give him some space. Initially, he will need to get used to you, so let him come to you. You will all need to get to know one another and he needs to learn to trust you.
How go we gain his trust? Just be calm, kind and reliable. Feed him at the same times each day, make sure his litter is kept clean, make sure he always has access to clean water, don't shout at him, run at him, flap your arms at him. Cats generally like calm people who give them space, but when they come and ask for strokes to make a bit of a fuss of them. All cats are different, of course, some are more gregarious than others. He may well hide under the sofa for the first few days - don't be alarmed if he does - it's quite normal.
How do we know if he's happy or anxious? If he's happy he will be relaxed, if he's unhappy or anxious he'll hide, run away, over-groom, possibly spray. Some owners find Feliway plug ins are helpful in the early days of getting a new cat.
Are cats ok with anyone stroking them? Um, not really! Cats decide who they like and don't, but just give him time/space and hopefully he will come to you and be happy for anyone in your family to stroke him. Try not to expect too much immediately though. Coming into your home for the first time will be stressful for him, so give him time to settle in.
Finally, the thing I'm most worried about is the litter, the poos the smell. We've been told to keep him in 4 weeks, then he should be allowed out and should toilet outside. At that point do we get rid of the litter tray? Remove poos as soon as he does them and either flush down the loo or put in dog poo bags and into the bin. The cat litter should help to control odour. As for getting rid of the litter tray - I wouldn't - as its a safe, clean place for him to toilet if he needs to or feels unwell when he's inside. If you plan to keep him in overnight its important to give him access to a toilet, just in case he needs it.