Love me some KEFIR! I've been buying mine lately at Aldi's; I think they have strawberry and strawberry banana flavors.
In the 1970s, we had home delivery of raw milk and dairy products from Mathis Dairy in Decatur, GA. That's when I was introduced to the wonders of Kefir.
It is an acquired taste though. How old is your son? Will he eat yogurt? There are lots of probiotic varieties out there that he might find more palatable than Kefir.
My son was literally weaned on yogurt. It's really easy to make yourself, if you're interested. You can control the acidity, tartness and fat content if you make it yourself. My son used to go through a half gallon of yogurt a week. He grew up preferring it to ice cream, making yogurt sundaes with applesauce toppings, wheat germ, etc. Thirty years later, he's probably changed his preferences to ice cream.