ACK! Please, please keep your child on formula or breastmilk until at least the age of 1 year:
http://www.cnn.com/HEALTH/diet.fitness/9906/23/cows.milk/...
http://moxie.blogs.com/askmoxie/2006/08/qa_introducing_.h...
And don't be fooled by a recommendation from your pediatrician - peds do NOT specialize (or even study much) in infant nutrition. Anyone who specializes in infant nutrition (or who is truly concerned) would never recommend switching before 1 yr, and after 1 yr, most would say to limit dairy to cultured products (yogurt, aged cheeses) in small amounts (think of it as a condiment like ketchup, not the main basis of the diet).
Don't foget that cow milk is from *cow breasts* and thus intended for *baby cows* -- it is SPECIES SPECIFIC. Cow's milk is NOT intended for baby humans - baby humans need human mother's breastmilk or if that's not available, then formula. The make up of cow's milk is NOT appropriate for human babies - formula is at least designed for human babies!
It is worth the $$$ to stay on formula for the next 3-4 months. You can afford that, or figure out some way to afford it (have friends who get free samples they don't want or use?). I wish moms understood that if you cannot or choose not to breastfeed, a baby really deserves donor breastmilk or infant formula for *at least* the entire 1st year. If the cost is a problem, please consider with future babies that you can give them breastmilk for FREE... free *all* the time, healthier than anything else out there (for mom & baby, amazing protective effects against many forms of cancer, diabetes), and perfectly formulated just for your baby's age.
Please keep your thriving son on formula, not baby cow food!