It helps a lot if others in his familiar circle of acquaintances, friends and family can all use the same 'stop' signals e.g. a hand held up to signal 'stop', and a clear word "Stop!". Then if he's stopped, they can give the next instruction for what sort of hug or kiss it should be. Not handy for strangers, but it can help us a lot with other people (she says, as someone well known for occasionally getting those boundaries wrong herself...).
Showering is a sensory overload of incredible proportions for many (not all) of us. I have to remember which bit of me is which in the shower and often forget bits, because I'm coping with the pain of the water and the pong of the soaps etc. The pictures of which bit to do are very useful.
I've heard useful things about getting a child to lie under a weighted blanket before a shower, or using deep pressure massage. It 'resets' our brain to remember where the whole of us is, and calms us down enough to be able to remember more in the shower anyway.
Weeing...wonder if he'd find it easier to hold it with a soft paper tissue round it rather than using his hands, if he can figure out how to wrap it in a way that the wee can actually get out? Just thinking out loud, really. If he doesn't like the sensation, (plus there's the loud noise of weeing and the smells of a loo etc), then it might help a bit.
Also, a target might help. I think some people paint a big dot on the back of the loo bowl to aim at.
I've managed to miss the loo and I'm a woman on the autism spectrum and sit down on the thing, so I don't know why I'm giving advice really
Eee, the things we have to do, eh?