This is a "multi-employer" pension scheme. Which means that he is on the hook for the collective shortfall across ALL the employers in the scheme, including much bigger companies that have subsequently gone bankrupt. This is not about him failing to live up to the obligations he made to his own employees. Which should be obvious given that the "shortfall" he is expected to pay seems to be many multiples of the annual turnover for a local plumbing business such as his.