Why can't he say
"No, thanks. Right now I don't need any more shirts."
For his stepdad, "XX, I appreciate your thinking of me, but I don't need any more shirts. Why don't you donate them to Goodwill?"
If anyone leaves the shirts, throw them away or donate them immediately. Don't even bring them in the house. I know there are clothing bins all over our city, he could find one that is close and drop off unwanted items on his way home.