https://www.callahan-law.com/are-non-competes-enforceable-in...
Specifically California Business and Professions Code Section 16600,
“every contract by which anyone is restrained from engaging in a lawful profession, trade, or business of any kind is to that extent void.”
In addition such issues must by law be decided in California courts and if they forced the issue into court and lost they would be liable for the cost of his defense.
Even outside of California there are limits to what you can enforce. Judges aren't liable to find that an infinite duration noncompete reasonable.
Another example in Washington State its now impossible to obtain noncompetes for anyone paid less than a rate of 100k per annum as an employee or 250k per annum as a contractor and they are limited to 18 months duration.
If you improperly assert a noncompete you are liable for 5000 or actual damages whichever is greater.
They are probably not asserting a noncompete because it is functionally impossible for them to do so. They would have to assert that he was making use of trade secrets or that in some nebulous way his design belonged to them. eg trade dress
https://www.findlaw.com/smallbusiness/intellectual-property/...
The answer is you need a lawsuit to decide but probably not.