I got these (arguably very hilarious) screenshots of his shirt, and it seems to have lettering on the side that says "____ Count on me" on it. No luck finding it so far, there seem to be few abacus t-shirts available online and the ones that do exist tend to have some of the most nightmarish graphic design I have ever been cursed to witness.
few stills of peter's other shirt. looks like it says "missing electron" on the side of a drawing of a milk carton as-well as a dorky quote underneath saying "gotta keep an ion it!" Honestly I think this one and the abacus shirt might be custom made for the movie. Few reasons I think this is just how easy you'd think they would be to find due to how simple they are but it seems no ones come close to doing so also the quality of the shirts seem pretty high end as it kinda looks like they printed the graphics on some blank James perse t-shirts as you can tell by the collars.