Maybe the other shirts were dirty. Maybe he’s thinking about you. Maybe it just goes with some of the other stuff he wanted to wear.
It sounds like you want it to be more. Work back into it slowly. Don’t expect things to be the way they were, especially if he’s the one that felt it was necessary to leave.
I wouldn’t read any significance into it. He might just like the shirts or be a grab the nearest clothes and put them on kind of person.
Guys don’t have the time (or money) to shop for new clothes. Wearing clothes from an ex is not a big deal. To them, it is just clothes and nothing else.