Tell him you can't afford to pay him, sadly, so you'll do it yourself. Not being an expert and it being quite tricky to do stairs it'll no doubt take quite some time. Certainly, though you'll start it on Sunday, you're unlikely to get it finished so you'll have to finish it off piecemeal no doubt.
Every time he contacts you to arrange a meet up, "sorry I'm decorating, darling" and repeat and repeat. If he mentions laundry, tell him, "oh you're welcome to come and use the machine and the iron whenever you want, you can keep me supplied with coffee while I work", sweet smile.
(Because clearly hanging wallpaper is work but washing and ironing is just so much fun! [hmmm])
Just wanted to give this so many upvotes, @Ifeelsuchafool is spot on.
If you don't feel up to sitting down with him and saying "I didn't think that was particularly fair, given that you live here 3 days a week and I wash, iron and cook for you, I thought this would be something you could help me out with, instead of just being able to give me a £75 discount on someone else coming and doing it." then go with the above.
If you don't feel up to doing the talking thing, you need to demonstrate that what you bring to the relationship has worth. If he doesn't get it by talking or by you showing what you offer, then you know what the relationship's going to be like and you can make an informed choice.