He gets results & it's all white hat (the only thing that could be considered a bit dodgy is the content writing but it's generally of good enough quality to be useful to a human).

He can point to previous clients whose traffic has jumped 2, 3 or 4 times over after he went through their sites.

People will happily pay $50,000 to make an extra $200,000. If he just said to them "give me $50,000 and I'll do your SEO", he'd be laughed out.