Let him play video games.
Let him take them apart to see how they work.
Teach him how to use HTML from the web, TONS of free sites, help sites, etc for HTML. It is the basics of all programing, etc. He can get into learning C++ programing also.
Find a local company that produces games or something similar and set up a tour so he can see what his office, work area, etc might be like when older.
Email a gaming company and see if you can't find like a "big brother" there that would be willing to email your son, talk to him in an Instant Message, etc. Someone he can talk to some to get ideas, stories of the industry, etc.
Look online and in any local school supply stores for any computer, programing or similar work books, class plans, etc. Do some "home schooling" since he is still young.
Find a college that offers gaming courses close by and pay a visit to campus, the classes, the work rooms. Let him talk to the students, etc.