I used to catch for my daughter who played fastpitch softball. She could throw hard and fast!(In fact, she was one of the few top B ball pitchers in the city). I also coached softball so I would catch for the other pitchers on the team as well.
At the minimum find a well padded coachers mitt and use a batting glove inside the mitt. Also get a pair of shin guards, they'll save your knees and shins. I found that I didn't need a mask/helmut because it can limit your vision, especially if you are coaching your pitcher.
I found that a good softball catchers mitt with an oversize pocket makes it easy to catch even a hardball. I used a SSK Medal Pro but just about any brand will do.
Good luck to your son, you are now officially a "Bucket Dad" (whether you use a bucket to sit on or not). Its great fun to watch your son/daughter pitch, especially if you work with them regularly. I took my daughter out 5 days a week for about 30-45 minutes. But softball fastpitch is not as demanding on the arms/shoulders as overhand pitching.
Just remember to let your son have FUN and keep the coaching comments to a minimum or you'll burn him out quickly.
Las Vegas, NV
"Every day brings a chance for you to draw in a breath, kick off your shoes and play your mandolin."