With regards to consistency, it's the key for young learners. However, your son will eventually be faced with a variety of English dialects sooner or later, so why not start introducing him to the different varieties of English now, especially while he's young and can still absorb the world like a little sponge.
Note that, Do you have? is considered American English, whereas Have you got? is considered British English. Mind you, I use both and I am North American: Canadian. Knowledge is power, right?
As for which text to choose next, your best bet is to go with what works; what your son enjoys and what you feel is appropriate for his level and learning style. After all, what's most important is your son's happiness.
All the best.
- For Teachers