You referred to the amendment in the past tense, so I assume it has already been adopted. The wording of the amendment will determine if the person is still an active participant as of 1/1/2015. Participation is not a protected benefit but the conditions for receiving a contribution are protected after those conditions have been met 1.411(d)-3 Q&A 1(d). So, unless the plan has a last day requirement, you may have a problem using another amendment to prevent him from getting a 2015 contribution.
If I read your situation correctly, the amendment, instead of increasing allocations for current NHCE participants, makes a short service ineligible employee a participant who receives an allocation. Does anyone know if the IRS would consider this similar enough to the situation in their short service memo to be a concern?