The military is huge, so it’s very possible for people to have different experiences. Most DIs are true professionals and won’t take things too far; that said, like any other large group of people, some aren’t so great. I can’t say it’s never happened that a DI has crossed the line. People in the military are just people: most are fine, a few are criminals. You can definitely find examples of people in the military being court-martialed for going overboard on discipline. Again, they’re the exception though, not the rule.
If your son joins, he’ll definitely have people screaming in his face and making him do pushups until he’s pouring sweat. There will be moments he’ll absolutely hate the people training him. He’ll have spent a ton of time carefully making his bed according to some very specific regulations, then someone will come by, decide it’s not perfect, and flip the mattress and tell him to do it again. But actually hitting him? No, I highly, highly doubt it. The vast majority are professionals who know the difference between training and hazing. They know how to take it right up to the line but not cross the line.