Supervisor eligible to OT?

On our letter of appointment, we have stated that “Your duties shall include overtime whenever the need arises” but we did not mention anything about overtime pay since all our workers are under non-workmen (mostly engineers and office staff with salary more than $2k).  
Recently we are going to employ a senior site supervisor with salary of $2,600.  His job duties are as follows:-
1)    Assist project engineer in monitoring the project site
2)    Attend site meeting and liaise & perform co-ordination works with sub-contractors
3)    Perform site-inspection, supervision of installation work & ensure that sub-contractors perform according to the project schedule
4)    Keep daily reports of manpower, safety issue and attend daily site meeting
5)    Attend site complaints and feedback to project engineer in order to resolve problems
When necessary, he is required to do some labour work in order to expedite the job & we estimated to be less than 20% of his job nature. As explain, I think the senior site supervisor position will fall under non-workmen categories (correct me if I’m wrong).
My boss has instructed me to draft a letter mentioning that there will be no overtime pay.   
Do you have any sample letter or clauses that mention this issue or what advice shall I give to my boss or the employee involved?
There is no sample letter or specific clause mentioned above. You may consider clarifying with your Senior Site Supervisor on the job prerequisite for him to work overtime.