If you have a non-solicit combined with a proprietary info agreement, it could stand up. The fact is, the tech would be using information gained from his employment to solicit clients.
It also depends on the employer and employee. If the employee leaves, prints some business cards, and tries to shoestring budget his business, they can't afford an attorney and will likely comply with any demands. The employer also has to spend some money on an attorney. If the employee gets some capital to start his company, he may be willing to legally fight it.
I suspect once any legal action is started, the tech would leave those clients alone.
It's also important to make sure the client understands the tech is a cog in the wheel, the wheel is what ultimately supports the client.
If a client is happy, they are not going to leave the first company.