Don't throw good money after bad.
If he isn't motivated now, giving him more money won't inspire him to work harder.
If he really wants a cut of the profits, let him put some skin in the game. Offer to share profits, but cut his pay. Set it up so that he has to maintain current performance to keep current pay. Work harder, bring in more $$ for biz=more $$ for him. Work less, bring in less $$ for biz=less money for him.