Step son refusing to shower

My step son is 14 and smells God awful because he refuses to shower. We are currently living with my MIL and my step son doesn't shower at places that aren't home. And since we don't have our home anymore he's refusing to shower. We are getting complaints from the school to about his hygiene. I ended up asking his maternal grandmother for help because before he lived with us he stayed with her a lot until she went into a retirement home. My husbands ex MIL has always been so sweet to me and she isnt afraid to admit her daughter wasn't a good mom. She had no advice for us sadly because its just how he is. He wouldnt even shower at his mothers house even though he DID live there and since then he wont shower in other peoples houses. It took us almosy 2 YEARS befoee we could get him to regularly shower at our own house. Im just stuck guys

Edit: I have asked him about it. He just says he doesn't know who will try to walk in on him and we are all family so that just confuses me. I said there's a lock on the door and he said the door isn't that hard to break open and I'm like "Why would we break the door Open?" Il