Buy a sub to 192.com - you can then search on the person by name and town of residence, and it will show all the companies of which he is a director. I'm not aware of any free service for this, though you used to be able to go to Cos Ho and look at the microfiche of individual companies for a small fee.
Does his employment contract specifically forbid him from any specific activities for a specified period after leaving the company? I've seen some pretty dodgy employment contracts over the years that do not/would not stand up in court. The phrase is restrictive covenant, and judges do not like them. They often throw them out wholesale. Far better to get the employee on taking/retaining proprietary information.