K.C.
Hi Z.,
Fantastic idea!
Check with local churches, especially those with MOPS (Mothers of Preschoolers--http://www.mops.org/). They can probably help in two ways. First, their congregations may be willing to contribute. Second, they usually have a very good sense of where needs are in their communities. They can find moms who may be edge cases--not qualifying for assistance but not able to make it on their own and relying on help from generous people like you and your son.
For receiving donations, maybe you could contact pediatrician and OB/GYN offices. It would be a means of reaching mommies who have piles of leftover or samples of baby formula at home and who probably have hearts to help kids.
Also, you might consider creating an "ad" for Craigslist or a similar site.
Best wishes!