Husband suddenly sweating profusely at night from his back?

My husband and i have a waterbed. we dont have the squishy under sheet that is suposed to separate the bladder from the fitted sheet. we have been sleeping on it since march and just randomly a couple weeks ago my husband started sweating literal puddles at night. he has never done it before and it just...happened one night. no starting to get sweaty, it just went from one night normal and the next night sweating buckets, but only his back...if that makes any sense and we cant figure out why or what to do about it. has anyone experianced this or know someone who has or just have any advice or ideas? cus seriously, we cant wash the bedding every day.