If this was not covered in the agreement whatsoever, then there might be an argument to be made that he is due market compensation or they are artificially reducing his earnout by depriving him of income he could make elsewhere. If it was covered and he agreed to it then that’s likely to be a problem.
However, these things are very difficult to negotiate unless you have some sort of leverage. If he can lose the earnout by not performing, or the business will not perform well enough to capture the earnout without him there, then he may have a very difficult time with it.
As an acquirer I have generally benefited from underperformance of retained executives. There are a lot of people like me out there. For that reason it’s best to have someone with investment banking experience work on M&A deals and not business lawyers.