I know some cats just won't use the covered boxes. Pouncer and Allen do, but Abby and Harry won't. They'd hold their pee for ever if I had only covered boxes. You can get high sided boxes where the wals are so high that he can't possibly fling the litter out.

As for the playfullness.... things are still new. Give it time between everyone. They'll learn how to "play nice" with each other.

Edit: also... how old is he now? I forget... that is also the position they take when trying to mate. He might be getting those hormones early, might be time for neutering already.