To wait might mean missing the boat.....You might want to draft him a letter explaining your situation and make him aware of your genuine interest. I would hold back on talking about terms(including how much you are willing to pay) until he shows interest in your appeal.
We prefer a letter as opposed to direct contact because he will hopefully keep the letter with your contact information on it.
The "Eckler Team"
Century 21 Almar and Associates
Venice, Fl 34285
As to what to offer, consider just saying "full value," not "full assessed value." An assessed value means nothing. Often, it's quite a bit less than fair value. And if your neighbor asks his son or daughter, or lawyer, about the letter, they may erroneously believe you're trying to "steal" the property. Also, check with a lawyer just to make sure that what you're presenting doesn't constitute a contract...unless you do want something binding. It may be your dream home today, but let's say in 5 years the neighbor wants to sell. He has a signed, dated document from you saying you will pay "full assessed value" if "he ever wants to sell." Now, I'm not a lawyer, so this isn't legal advice, but be careful. True, contracts require some form of "consideration," usually a dollar amount (unless the transaction is between relatives, in which case the term "love and affection" is often the "consideration"). However, it could be argued that the document itself has a value, thus satisfying the requirement of "consideration."
Not trying to scare you. Just suggesting you phrase the letter carefully.
Hope that helps.