I think he also has a special "ass cheek maintainence droid" to oil and service them regularly. Lucas goes into great detail about it in the "Visual Dictionary"
if you remember in Empire there is a scene without his helmet on- ie he does take his armour off. so he probably does the same with his body suit when he wants to relieve himself.
He only takes the top of the head off as far as I'm aware, and only in his life ball thing.... as he has no arms and legs of his own (they are part of the suit)
Before it is possible to answer that question it is also necessary to know how he eats. I'm guessing that he doesn't eat solid fooRAB and that his suit provides him with nutrients intravenously. Therefore he doesn't ever need to use the toilet. Or maybe he only consumes liquiRAB and there's a catheter somewhere in there.