
This is a BIG topic in other threads. The short answer is: Commercial feed is almost always cheaper and better quality than you can make yourself.
There are certain amino acids referred to as "limiting amino acids," meaning, if you don't get enough of those in the feed, the birds will suffer. One of them, Methionine (often referred to as "Met" for short), is critical to physical development, especially in chicks. If they don't get enough Met when they're young, there's no way to make it up and correct the problems that can ensue.
There are very few plant sources of adequate Met. In fact, in organic feeds, a certain amount of synthetic Met is allowed, just because otherwise, the feed would be deficient in this critical amino acid. And that's just one of the critical amino acids.
Yes, you could feed your chickens leftovers and let them forage. They might be fine. They might be fine for a while. They might have health issues right away. It's hard to tell.
In the tool and die trade, we used to say, "Cheap, Fast, Good. Pick any two." Meaning, if it's cheap and fast, it won't be any good; if it's cheap and good, it won't be fast; and if it's good and fast, it surely won't be cheap. (For "fast," in this case, substitute "easy.")
There is a Cheap, Fast, Good option for feeding your chickens: Commercial feed. If you can find ways to stretch it, then it's even cheaper.
I'll be stretching out my feed by gathering Japanese Beetles when they arrive. But I will spend an hour a day collecting them. I can give my chickens great entertainment and some food by giving them a bucket of weeds from my garden. Another time commitment.
Please talk to your dad about this. Pick a time when he's not stressed about something else. Maybe write down what you want to say beforehand. Offer to pay (or work for) some of the feed cost.