US visa for a child

Yes, he needs a separate passport for sure. Not sure about the visa; best thing you can do is call up and find out.

But from the belgian usa website:

http://belgium.usembassy.gov/faq4/traveling-with-children.html

My children are included in my passport. Do they need their own passports to go to the US under the Visa Waiver Program?

Children, even infants, need their own machine-readable passports for travel under the Visa Waiver Program.

and if you wonder what does infant mean:

The term infant is typically applied to human children between the ages of 1 month and 12 months; however, definitions vary between birth and 3 years of age. A newborn is an infant who is only hours, days, or up to a few weeks old.

You can go to your own country's usa website and also see what it says about the visa...