That's what the 1099-NEC is made for, income paid to independent contractors, ICs are self employed, self employed people pay self employment taxes

But did the issuer use the correct form for the situation?  

Why did you post 3 different posts re the same question on this board?  If you need to change the wording, there is an edit button you can do after the post.

The income was from serving as a trustee to his union.  Can it be reported as other income not subject to self employment tax?

You've asked three times about 1099-NEC. Because it's scattered in so many places, you have info that matters, but that info isn't here:

"My client received a 1099NEC for serviing as a trustee to his union."

This is likely what you have:

When the person is not an employee of this entity, not under their direction or control, that makes them an Independent Contractor. This 1099-NEC is filed as Schedule C for the person's 1040. Your person would be a Director, not an Officer, in other words. They don't represent the union or its members; they guide the board for governance of the entity.

Thank you, whoever combined topics.

Also, if that union is a 501(c) organization, then the IRS is your friend:

https://www.irs.gov/charities-non-profits/exempt-organizations-who-is-a-statutory-non-employee

As a professional tax preparer, you need to utilize the IRS resources for the most current and applicable info.

Maybe:

From the Schedule C Instructions - look at the part I highlighted.

"An activity qualifies as a business if
your primary purpose for engaging in the activity is for income or profit and you are
involved in the activity with continuity and regularity. For example, a sporadic activity, not-for-profit activity, or a hobby does not qualify as a business. To report income
from a nonbusiness activity, see the instructions for Schedule 1 (Form 1040), line 8i."

You and your client need to decide and document whether he meets the above definition of a business.