You asked for advice about this almost exactly a year ago, and at that time my advice was that before you embarked on a protracted domain name dispute procedure you should get a solicitor to write to this man.
The basis of your claim to the name is that the third party has no rights or legitimate interests in it, and that shouldn't be too difficult to demonstrate. Presumably the name reflects the name of the charity, and as you have been running a website on the name for some time your claim is substantial.
You may have documentary evidence to show that when the name was first registered the person concerned was a volunteer, and my claim would be that as such he acted as the charity's agent when he registered the name. He was acting on your behalf, not as a private individual, and now that he is no longer associated with the charity he has no claim to the name.
Hit him with a solicitor's letter - it may do the trick, and if it doesn't you have the ICANN Uniform Domain Name Dispute Resolution Policy to fall back on.