He's the only employee in a 2 man company. How exactly do your think the relationship here is likely to be play out? IMO it is likely that he has a pretty good and probably rather personable relationship with the company owner. And quite likely has rather good bargaining power already given that he can double his employer's workload by walking out the door and it'd in all likelihood be a big headache to replace him.

If he can't leverage his power when he already represents 100% of the company's employees a union is unlikely to help.