Ugh, that's not good. He's registered self-employed as a sole trader? I've seen a few people have got that message when they believe they should be eligible so perhaps there's a glitch or something.
This was the criteria they published previously:
To apply, you must be a self-employed individual or a member of a partnership and:
traded in the tax year 2019-20 and intend to continue trading in 2020-21
have trading profits of less than £50,000 a year
earn the majority of your income (ie 50% or more) through self-employment
have filed a tax return for the 2018-19 tax year. Anyone who missed the 31 January deadline has four weeks from 26 March to file their 2018-19 return and benefit from the scheme.