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.
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?