The soil idea is a very good one as this is what he's used to.
Until he's neutered, he may spray, but this is to be expected. Try to avoid it, because it will encourage him to spray in the same place after he's been 'done'. If he does spray, clean it thoroughly with biological washing powder to get rid of the smell.
To encourage him to use the tray, scratch the soil/litter with your finger while he watches. This will give him the general idea. GENTLY sit him in it and push his bottom down slightly (being careful of his wounds). Then leave him.
He'll get the idea that he's supposed to use this as his toileting area.
If he makes a mistake, don't shout at him, just put him in his litter tray and make soft encouraging noises and stroke him. If you do this every time he makes a mistake, he'll realise where he should go. When he uses the tray, make a special fuss of him and give him a small treat.
In time, he'll learn. Don't expect him to know what to do too soon. Give him time to learn how to be your cat.