File an OSC/Motion with the CSE, asking the judicial officer to disclose the CP's address for purposes of service of process for a custody hearing.
Most CSE agencies will accept service of process or agree to forward the paperwork to the CP. You must have a hearing in the works. (They have to cooperate with service of process to the CP).
Before you try this, contact all of the CPs friends & relatives to see if they will give you info (you will be asked by the CSE court as to what efforts you exerted to get the info). If you have exhausted all other means of getting the CPs address, then the CSE should assist with service (not necessarily giving you the address, just forwarding the paperwork & providing a proof of service for court).
That will, at least, get you in 'custody' court. Then, the 'custody' court can try to resolve the problem, if they choose (appointing an atty for the minor & instructing CSE to give the address to child's atty, or other means).
A court may not help in this situation, as it's been '6 or 7 years' since he pursued visitation.
As Blonde said, try ChildFind or other search methods.