In this case, perhaps it is better to contract him directly, and not through a third party.
It generally would depend on how deep your pockets are. If you are Microsoft, then you have deep pockets, and a seller might try to get much, much more than the domain is worth. That's a good reason for going through a third party.
In this case, you are a non-profit. You aren't Microsoft. Unless it is well-known that you are a wealthy organization rolling in cash, do it directly, and perhaps the guy will get a good feeling about it.
I would also name a figure in the initial contact. Make it generous. This will wake him from his grave.
There are posts here that go into detail about how to value a domain name. I would read those, and come up with a fair value. You will have to up it, of course, because you cannot assume that he has any reason to sell, so you will have to offer more than fair value.
You are in luck in this case, as you can probably estimate his traffic based on your own, and a guesstimate at the percentage of users that would try to direct-navigate to your site. (You should be able to find some published statistics on this.)
Having been on the other side of this equation, I think naming a generous price in your first contact is key.
It goes like this:
(Inquirer #1): Do you want to sell your domain?
(Me): How much?
(Inquirer #1): $100
(Me): No, thanks.
(repeat a couple of times)
(Inquirer #4): Do you want to sell your domain?
(Inquirer #5): Do you want to sell your domain? I am willing to offer $400.
(Me:) I'd like $5000.
(Inquirer #5): Sorry, thanks.
(Inquirer #n): Do you want to sell your domain? I am willing to pay $nn,nnn. Here is my phone number.
(Me:) (Is this guy serious? Is this a joke? OK, I'll call him and see what's up.
If he had not given a fair and generous price and his phone number, I would have just ignored the email. I suspect this is pretty much the way it goes for most.
If you can get a phone number (by hook or crook) call the owner - don't use email. Or send a letter if you can get a real address.