How will family business be affected by divorce? He is listed as sole proprietor, business was started after 20+ years

It depends on many factors such as whether he owned the business prior to marriage or if it was strictly accumulated during the marriage. It should be treated as marital, but then we have to determine to what extent is it marital. Was the business gifted to him? Did he inherit it or a portion of it? Are there now partners to consider? You will want to look at whether the business itself has intrinsic value, or whether the values strictly comes from the business assets.