Have you tried sitting with him for some of the lessons, to see exactly why he's having problems understanding? With Teaching Textbooks, I have the children watch the cdrom first, then read the lesson in the textbook itself and work the practice problem. Then they check the practice problem to make sure they get it. If not, we watch the solution cdrom and watch the lesson again. Next they complete the assignment problems. A student who is struggling will do only the odd problems (leaving the even for the next day.) We then check the problems and watch the solution cdrom for all the ones they got wrong. We've been using TT for three years with one math-kid and one non-math-kid (High School level) and so far this approach has been successful. I do require them to slow down and really analyze when they're having a problem. We try to be very mindful.
Perhaps if you help to really focus his lessons it'll help. Most 14 year olds need a lot of "help" to focus! ;-) Totally normal! It's so expensive, I hate to see anyone curriculum-hopping if they don't have to! :)