Hi MissPiano and
to the forum.
I'm more a violin teacher than piano, but the finger number problem still happens! Generally speaking, as soon as I am aware that there is beginning to be an issue with reading of notation, I give the child a set of flash cards to take home, and explain - preferably in parent's earshot - how he should use them on a daily basis (parent testing child - but child can test parent too!) Make sure the note name is on the reverse, in case of musically illiterate parents. With a child with the problems you describe, I would just ask him to learn one or two (say middle C in each hand to begin with). Agree with him that he can have finger numbers except for those two notes. Get him to write them every lesson for you, because that will reinforce the reading. When he's confident and recognises them 9 times out of 10, add one further note.
As Piano Meg points out, Me and My Piano has quite a lot of fingering printed in. When one of my sons used this book, we tippexed out some of the numbers so that he didn't get too reliant on them! Little and often is the way to go, and make it into a game for him so that his interest is rekindled. Could you try a giant stave marked out on the floor with masking tape? Or a matching game with musical snap cards / dominoes?
I would also introduce (if you haven't already) another book for him - Dozen A Day Mini Book begins with just two notes, and it might help to use this alongside Me and My Piano to reinforce the note-reading. A theory book, too, would help him to consolidate.
I have taught several dyslexic children, and it can be frustrating, because just as you think they've got something, suddenly they lose it again. It can also have completely different challenges for different children; no two children with dyslexia will have the same problems - even two dyslexic siblings I teach have markedly different problems. Enlist the help of the parents to help him with practice at home and (if it helps) use coloured overlays for his music.