Hes probably been exposed to a low level back in Feb and has immunity.. It looks to me like there will be a test coming along. You take it and the results will provide a pointer to how you will suffer from COVID if you get it. All the way from - likely to end up in ICU through to asymptomatic. Based on the T cell response. Its also interesting that some people anyway have T cells that recognise COVID-19 .. and that it might be that very similar things may have been through east Asia during peoples lifetimes.