Seeking advise from parents of autistic children, my son with autism REFUSES to keep a fitted sheet on the bed?

ive tried a dustmite cover that the matress fits into and zips all the way around which he ripped,tore off first night. tried a quilted cover that looks like a mattress top hoping it would seem like the mattress to him, which he wait for it....takes off, i still put a fitted sheet on every night but for several years he just takes it back off and he has an allergy to dust mites so im not happy about him INSISTING on sleeping on the bare mattress. any thing you have tried with your child with autism that has worked?