I agree with the point that many small start-ups fail once people realise the work is not just the trade itself, but also pricing, sales, scheduling, payroll, client management, insurance, compliance and cash flow.
That said, if he is genuinely one of your best people, I would also look at the retention side before making it purely adversarial. A strong employee usually starts thinking this way because they want more control, more upside, more recognition, or more say in how the work is done.
The conversation does not have to start as a hard one. I would separate the issues clearly: support the ambition, but set firm boundaries around availability, performance, client contact, use of company resources, and anything that affects your business.
If you value him, it may be worth showing that clearly before assuming the only options are confrontation or losing him.
