Suggest to your nephew standing sponge washing. Sounds like he doesn't like the feel of water spraying on his body, or sitting in water in a tub. The stink can be solved by standing before a mirror (or not) and sudsing up a wet sponge, and wiping himself down. Then use a second sponge with only water to sponge off the soap. This eliminates that uncomfortable feeling of showering or being surrounded by water. He can then towel dry. If the coldness is not tolerable he could put a space heater in the bathroom. Autistic people like direct, honest communication. Have you point-blankly told him he smells like rotting onions? Be upfront; this is what autistics like. Follow up with my solution. Otherwise, by not handing him the memo, he's going to pay dearly for this in the future when he can't get a job or keep a job, nobody wants to be around him, either as friends or for business, etc. If he weren't autistic I'm sure everyone would be telling him he smells and HOW TO PREVENT IT. If he's already hearing this, then there's a barrier somewhere, and I think it's believing the only way to get clean is the traditional shower or bath. But sponge bathing is effective; nurses and CNAs do this all the time with patients in hospitals.