I had two PPs come from Petsmart in Tigrina (??) shells which are for micro/tiny crabs who like D shaped openings and Kaylee seriously wore that thing through two molts until this last molt (her 2nd w me) when she doubled almost tripled her size and tbh her head is bigger than that opening. She wasn't giving up her odd choice in shells for nothing. Anyways, odd crab stories aside.
Have you ever thought of keeping his preferred shell in sizes kinda-way too big? I'm thinking his bum wasnt comfortable in the, usually E, shell. Some crabs stay in shells seemingly too small specifically because their abdomen (bum) fits better. Others (which probably isn't the case because he seemingly fits perfectly in the one he doesn't prefer) go in shells way too big for their bum because they have an abnormally big claw. All depends on what that specific crab feels deserves more priority tbh.
In conclusion... Silly crabs
Mommy of 10 PP babies:
Poseidon (Sid), Toby, Matty, Junior, Aries, Kaylee, Turbo, Adrian, Pepper and Flower. RIP Octavius & Fred