P.M.
You've gotten some interesting answers that may give you some help, but I can see a potential for most of them to backfire, too. "Ruined" toys get thrown away? He'll probably choose to pee on things that you can't/won't throw out, like carpets, furniture and clothing. Putting him to bed early? Perhaps he actually needs more sleep, but if he doesn't, what's to keep him from peeing on his bed in protest?
Having him clean up after himself is probably the best idea, but I'd put a VERY careful check on myself if I were you to present this consequence in a completely calm manner, perhaps even help him cheerfully with the cleanups. This may not only take the steam out of his activity, but give the two of you a way to bond, and help assure him that no matter what he does, you love him and want him in your life.
All behaviors are strategies for meeting some perceived need. (This is true for adults as well as children. You can ALWAYS find some need driving a behavior if you look for it.) I would imagine your son has a great deal of anxiety about how permanently you will be in his life, as well as possibly grief for the other family members he had come to rely on emotionally. In addition to wanting to have his own way, he could be testing you to see whether you will leave him again. I've seen angry, defensive, and "rejecting" behaviors in populations of children who have experienced rejection or deep loss. After they satisfy themselves that the adult will hang in there and care about them, much of that behavior subsides. (This can take awhile.)
You say he's a loving little boy. This is wonderful – this is the child who wants a connection with you. If I were in your position, I would put most of my energy into two responses.
First, noticing when he does cooperative, loving things. Don't reward, and don't praise excessively for his behaving properly. Just notice out loud, "I see a boy who helped clean his room … came when I called … gave me a sweet smile … got dressed by himself …" etc. Or, "Those are some big feelings you've handled there … That was such a grownup way to handle that situation …" etc. Just notice with a friendly voice and a smile, and at least sometimes, a thank you. This may take some real effort on your part if this doesn't come naturally. But a loving child longs for connection and approval above almost anything else, and when you can give him that, he'll start finding ways to get more of those strokes.
Second, I hope you'll get the wise and practical book How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. This book is a gem, overflowing with real, encouraging examples of how parents helped set the conditions for the child to address the problem himself. And though we don't usually think about young children in these terms, they can be creative problem solvers. Plus, kids are more invested in solutions they think of themselves, and are more likely to work at making them a success.