Aside from food stamps, there are several other services your family should qualify for. WIC until your son is five for help with food and HUD housing for rental assistance. Early Head Start now and regular Head Start once he's three will cover your daycare and preschool expenses until he goes to kindergarten and also provide dental, vision and developmental screenings. If you want to go to college, low income parents qualify for all kinds of grants and other financial assistance.
You can buy you son all kinds of toys and clothes for very little at Goodwill, The Salvation Army, and garage/church sales. I'm a preschool teacher and I find the majority of my classroom toys that way. Last week, I bought a Little Tikes piano, a Fisher Price light up baby tball set, a set of wooden alphbet blocks, two toy baseball helmets, a box of matachbox type cars, Two pairs of jeans with the tags still on them and a used but still in good shape sweater for my step daughter. I spent less than $15.00
Take a lot of trips to the park or nearby lakes. Aside from the gas in your car, these are free. Pack lunches and have a picnic. Find out if the zoo has any special family days with reduced admission. Our town also has free kiddie pools in two of the city parks.
There are a lot of things you can save around the house that your son would love to play with. My preschoolers LOVE thier toilet paper tube binoculars. We eventually had to make a set for each kid to avoid fights. They also spend hours sorting food colored noodles into empty plastic food containers and sorting different bottles caps/lids into piles. After Christmas, they cut apart used wrapping paper and ribbons. Just google seach "preschool websites" or "preschool lesson plans" There are a bazillion free websites with lots of good ideas. (my favorite is everythingpreschool.com)