My only concern (probably unfounded) with starting him on a short scale is that he may develop "bad" muscle memory and that he would have to readjust later to a full-scale instrument... Am I being paranoid?
Yes, you are being paranoid. Here's why.
Classical string instrument training starts very young. Some training regimens such as Suzuki, start at 4 years, and are designed to begin before the student learns to read. These young players start on very small scaled instruments and may have 4 or more intermediate sizes of instruments before they reach the size they will play as an adult. For example, upright bassists of that age sometimes sometimes play small restrung cellos before venturing into even the smallest size basses, and Cellists of that age often begin on cellos that are not much bigger than full size Violas.
What I am trying to say is that short scale is definatley the way to go. In fact, if you are going to start him before the age of 10, I would recomend a scale even shorter than 30". Every child is different, but 30" is massive for a youner child. Learning on an instrument that is too large for a student's stature will cause many problems that can be avoided with an appropriately scalled instrument.
Also worth considering, is training in a classical instrument when very young, and introduction to electric bass later. Any of the stringed instruments (especially double bass) have lots of transferrable principles to playing electric bass. My brother started on Cello and I on upright bass and in middle school he added added guitar and I added electric Bass. Having a solid musical basis helped us emmensely.
Hope that didn't get too far off topic.
