You can buy them with fleece on the outside and a plastic membrane in the middle. I cant remember the brand name though... but this might be worth a look at http://www.johnlewis.com/230965267/Product.aspx. Another thing is that if you put the waterproof one on top of the mattress and then put a fleece blanket on top of that and then a sheet that would stop him from getting as hot ? I think that the fleece is waterproof as some people use them for nappies. Have you thought about one of those sleeping bags so that you dont have to put such a thick duvet on?
I made the mistake of accidentally washing one of the bedmats a while ago, it was a nightmare, imagine if you had put an entire box of tissues in the washing machine, never seen so much white fluff in my life.