Yes, but be careful in drafting the match provisions and the timing of when amendments can be made to the plan. For example, if the SHM is described broadly as discretionary, then the plan would have prescribed notice requirements and could be subject to the "maybe SHM" rules which could affect all participants; or, if the plan has an SHM for NHCEs and discretionary match for HCEs, the match levels and match rates together could break the safe harbor. Also be mindful of the plan design if the intent for the SHM is to be an ADP safe harbor or both an ADP and ACP safe harbor.
You may want to determine the goal the parents have in mind for contributions made to their son's account. For example, if they only want the son to have more contributed to his account, they may be able to increase his salary and have him increase his deferrals while still excluding him from the SHM. This could get the son to the same place without complicating the plan design. If they need to justify the salary increase, they can increase his hours or his responsibilities.
Sometimes the plan sponsors can be myopic and they do not consider easy and available alternatives that can get them a desired result without messing around with the plan design.