Congrats to your son! I know the feeling from my son's soph season last year until he got the good news. It's nerve-wracking for us parents; I can't imagine how it must feel for the boys themselves.
Tell your son, he's earned himself this opportunity through his hard work. Enjoy and celebrate this moment! But now the real hard work begins, in making the most of this opportunity. My son only pitched a few innings and DH'd or pinch hit last year. But he earned a reputation as a clutch player, and that has paid huge dividends. Most importantly, he learned an invaluable lesson about doing whatever the team needed him to do, and that has stayed with him even off the field.
Best of luck for a great season!
PD