Hi docs, would working out my chest decrease my pectus excavatum?

No. Pectus excavatum is a disorder of the costal cartilages, causing the central portion of the chest, or sternum, to be sunken. Your chest wall muscles are over the ribs, not the sternum, so no amount of exercise will make the pectus decrease or go away.
Pectus excavatum. Pectus is a structural abnormality of the skeletal system and is unlikely to be changed materially by working out.