My son who is now 10 and dyslexic had the same problem.
He finally learned his letters and numbers through multisensory ways. Try making the letters/numbers in clay, writing them in sand, finger painting them, writing them in the air and trying to imagine them in different colors. Montessori uses sand paper letters.
It took my son a long time, but he eventually learned them. I would make sure to make it fun with lots and lots of positive reinforcement. Learning should be fun.
The reason my son had such a difficult time with letters/numbers was weak symbol imagery. Even though he eventually learned the letters and sounds, he had trouble recognizing words. Lindamood Bell helped with this with their Seeing Stars program. (Lindamoodbell.com)