You didn't give enough info yet for anyone here to compute his basis. Clearly there were losses or distributions in prior years that created that negative balance in retained earnings, but we don't know if aggregate prior year losses were enough that stock basis has been reduced to -0- by the start of the year you are working on, and if there was a reduction in debt basis prior to the BOY. Was debt basis at BOY a reduced amount below 100% of its value?
Basically, there is stock basis and there is debt basis. Once stock basis is zeroed out, the shareholder can continue to deduct losses on his personal return up to the amount of debt basis. If both have been fully used, then his basis is truly -0-. If the debt basis had been reduced below its face value, any repayments of the loan that are paid to the shareholder might create taxable income. The character of that income depends on the type of loan. The shareholder's basis doesn't have to be -0- to create this income, only that debt basis had been previously used (reduced) when he deducted losses, and if repayment of the loan is made before the debt basis is restored to 100% of face value, then he will have income.