I assume either the son owns the home or he owns the home jointly with his parents. If that is the case and the home is his primary residence (even if it is not the primary residence of the other owners) it would be listed as owner occupied.
Reg C, Appendix A:Owner Occupancy. Indicate whether the property to which the loan or loan application relates is to be owner-occupied as a principal residence by entering the applicable Code from the following:
Code 1–Owner-occupied as a principal dwelling
Code 2–Not owner-occupied as a principal dwelling
Code 3–Not applicable