jbbaseball--playing may not make it worse, but by playing, your son risks another injury---which may sideline him for a longer period of time.
My son, also a middle infielder, tore his labrum at the beginning of the season in his junior year of high school. We didn't know that it was torn, only that it hurt. But because he didn't want to lose his starting shortstop position, he "played through the pain"...which developed into elbow tendinitis. And then he couldn't play. The doctor didn't want to do the surgery right away, but suggested that he rehab it; sometimes the rehab prevented surgery. But the way, the two MRI (a regular one and one with the contrast dye) were inconclusive, as they often are. The doctor, being an understanding White Sox team doctor, allowed him to stay in the game and allowed him to hit--as the eh or dh. After not playing for a while, he finally got his swing back, was hitting the ball very well and then oops--he got hit in the wrist, which broke both bones. Bummer. This delayed the shoulder rehab leading to the surgery, but it was finally done in mid September. The doctor promised that he would be ready to play in the spring.
Until the rehab in October was delayed because his wrist hurt and continued to be swollen. So off we go to the hand specialist, who very conservatively wanted to rehab what turned out to be torn cartilage (torn when the pitch broke his wrist bones) before considering surgery. Which was determined to be necessary in February--and it was performed on the morning of his high school try-outs in his senior year. Another bummer.
So although playing may not make his shoulder worse, it may keep him out longer with a more serious injury. Freaky as it may be.
Good luck with best wishes to your son. Feel free to pm me to talk about this....I really do feel your pain and dilemma.