With my difficult child, the only thing that worked has been rote repetition.
A laminated checklist of all the steps for proper washing, posted on the wall in the bath. A washable marker attached, so he can tick off the steps as he completes them.
Baths not showers. It's easier to fake a shower than a bath.
husband or I, and now Residential Treatment Center (RTC) staff, watch him run a bath and put bubble bath in it. Like Marg said, soap leaves a scummy ring. Bath bubbles clean the boy and the bath.
Minimum time in the bath 30 minutes. That way, even if he just sits there, the hot water and bubble bath will have some effect.
Post bath, he puts on a pair of shorts, bathing suit or boxers, and submits to a full inspection, as follows:
1) Evidence that he actually got in the water and stayed there. Wrinkly skin on fingers and toes, a scrubbed look to the rest of his skin
2) Sniff test: Pits, feet, hair, breath
3) Visual inspection: Ears, between fingers, between toes, back of the neck, soles of feet. Anywhere that grime is likely to accumulate, and not come off without actual washing
If he doesn't pass muster on any of the inspection criteria, it's back in for a do-over. No arguments about whether he did or didn't wash at all. Simply, "difficult child, you didn't wash properly because you're still dirty and you still stink." Run a fresh bath and try again.
The record for do-overs was 5. difficult child had to take 5 baths, he was starving and I even let him take a plastic bowl of dry cereal in with him to eat while he washed. I wasn't ready for a dinner battle on top of the bath battle.
Because he has to follow the same routine every night, he has gradually gotten the hang of it, and tends to bathe properly most times. If there's ever a break in his routine, such as an overnight visit home, the whole thing falls apart, but he regains the skill much more quickly now.
As for toothpaste, I agree with Marg that you can substitute. Personally, I make a paste of baking soda and a bit of water and use that. Makes my mouth feel very fresh and clean without the over-strong minty-ness of toothpaste or all that froth. Also whitens teeth.
This is a tough nut to crack. Persistence seems to be the key.
Trinity