I.M.
S.,
is he going to speech therapy because he had ear problems and couldn't hear right, therefore not pronounce the words correctly or for another reason?
My oldest was in speech therapy as well for the reason that I just asked you, too many ear infections until he got the tubes in. But that didn't seem to affect him with the potty training.
If I was you after you are done with that last book you have and you don't see any changes in him; I would take him to the doctor's and have him tested. Maybe it is something that is inhibiting him from getting the swing of things. You do what you can, but let the doctor's do their part too. Don't go crazy and lose you sleep over it. Some kids are different and take longer than others. Maybe there is nothing wrong with him, and he just needs more time.