I could get by on that if I had to.
Store cost of flour is less than dollar per pound and is about 3.5-4 cups, which will easily make a dozen biscuits. Get the self rising and save on added cost. If bisquits are not a skill he's developed, then use half corn meal and make cornbread instead, something almost anyone can do. Indian Head Yellow Corn Meal two pounds for $1.49
A 5 pound bag of Russet potatoes in my area are $4. One pound of microwaved or boiled potato can be quite filling.
Carrots are $1 per pound. Add potatoes above and some flavorings and have a stew.
We pay $1 per can for Giant Food store brand chili with beans, and that's 15oz cans. I had one just today for my lunch, some homemade hot sauce (from my cayenne peppers grown this year) on top of it.
.Aa 15 oz can of Contadina Tomato Sauce or Hunt's at $1.29 will easily cover a $1 pound of store brand pasta, or $1.70 for Barilla or San Giorgio. That's quite a meal for under $2.50
Beans, such as Chick Peas, Pintos, Lentils, Navy, Black beans, all run from $2 to $2.50 per pound. Buy your son a crock pot and teach him about the lazy way to cook a filling meal. Combine with rice.
Giant store brand rice is $1 per pound for long grain white rice. Mahatma rice is the same price per pound in 5 pound bags. If he wants it more flavored go for the Rice O'Roni which is $1 for just under half a pound in the box. I used to live on that and Ramen noodles when I was unmarried in my 20's!
Oatmeal, it's under $2 per pound in the large Quaker Old Fashioned round box. I eat it "uncooked" with milk and sugar (or sweet & low) since it's already been steamed, cracked and rolled, then lightly toasted. If he can get down a pound of that after letting it soak up some milk then he'll not be hungry all day. I know!
There's no excuse for not being able to fill up on $3 per day if one is diligent about what he purchase and what he eats. Yes, means you don't frequent the fast food places and might cut back on meats and cheeses, but you can still be filled each day with just the smallest effort and economy of purchase.
These are all store prices from my area which is supposedly higher than other places than in this Baltimore to DC shopping area. I used Peapod online shopping for these local prices, it's a home delivery service provided by Giant Food stores in this area. You can check them there.