I would say a 23" road bike for a 5'7" rider would be a bit on the big side. If you go with a 52 or 54cm (21 to 22") frame, it should be big enough now, and still offer some room for growth (I'm 5'10" and my 2 road bikes are a 54 and a 55cm)
If you don't know what type of cycling he's going to like in the long run, and he's happy with your hybrid, then continuing with a hybrid is probably ok for now for going on rides with you. Then he can think about specializing when he starts going on off road adventures in the wilderness, or long road rides to neighboring cities, or doing stunts off jumps, or whatever floats his boat.