Give him "Einstein Never Used Flashcards". It's full of good reasons not to do this. Children this age must play to learn.
There's a fairly academic book called "Play = Learning" which is chock full of research studies that show that formalized learning early on is a bad idea, if he needs more than Einstein Never Used Flashcards.
Since I'm reading the book in my spare time, I'll give you a quote:
"The data are incontrovertible. They have been telling the same story throughout the last 40 years of resesarch. When children are in environments where learning is occurring in a meaningful context, where they have choices and where they are encouraged to follow their interests, learning takes place (Hirsh-Pasek & Golinkoff, 2003). Ironically, as Hirsh-Pasek and Golinkoff argued, we have adopted a metaphor of the child as an "empty vessel": pour in the facts and the child will passively absorbe the material. However, the research tells us exactly the opposite. In preschool, when children are pressured to learn in schools with "academic" as opposed to developmentally appropriate curricula, they report being more anxious and perfectionistic (Rescorla, 1991) than their more playful peers. They are no more ahead in first grade achievement. Such programs also have the effect of reducing children's motivation and making them have lower expectations for their academic abilities, less pride in their achievements and more dependency on adults (Stipek, Feiler, Daniels, & Milburn, 1995) -- regardless of social class."
(Singer, Hirsh-Pasek & Golinkoff, 2006, p.6)
