There are indeed a different set of rules for VAT for farmers. You need to quantify how much VAT that is being paid on the farm. Setting up and detailing with annual compliance for a company can be expensive but if he can save VAT then it may be an option. There are a number of issues that need to considered before he deregistered, like plant that VAT was reclaimed on.
There may be seperate tax benefits from forming a company for pub trade.