There is not a whole lot that they can do to the machine, nothing that you couldn't do accidentally, just have him be careful with pins, the worse danger. And if there is a speed control, turn it down so he can't go that fast, I have taught all three kids to sew, even my son who is currently 13. He was learning the big embroidery machine the other day. If he is interested, now is the time to catch him. you obviously know that there is some really easy things that he can start with and get good results to help keep him interested.
I used to sew with the wee one on my knee for a lot of years before she started.