Carpenter's Union Vacation, holiday and sick leave, with dividends . . . taxable?

My son belongs to the Carpenter's Union (California).  His employer pays into the Carpenter's Union Vacation, Holiday, and Sick Leave Trust Fund.  Nothing shows up on his W-2.  The Trust Fund sent a check to my son for the accrued amount in his account and also pays dividends of the earnings on the cash.  With the check is a letter saying the dividend portion may be taxable income for 2021.  How do I handle his $5000+ in vacation, holiday and sick leave, and also the ~$300 in dividends that were included in the check?

 

Thanks so much for your advice on this!