First question is, how valuable is the domain to you?
Any offer will have to be enough to allow him to make some profit from the domain registration, possibly hosting, and perhaps a small incentive during the holidays for profiting on an unused domain. If the name is an actual word, he's going to assign some inherent value which might make the domain more valuable to him. The fact that you have some back and forth dialog means he's at least considered it enough to respond.
In cases like this, I generally let them suggest a price and use the 73% rule. Generally, they won't accept less than 73% of their initial offer. Once you have a number to work from, you can start to negotiate the purchase. If the answer exceeds what it is worth to you, you could try a hail mary and say, well, we really only had $x budgeted, thanks for your time. If you ever reconsider, please let us know.
Money today sometimes exceeds the possibility that he could sell the domain in the next year and make more than that, especially if you're the first offer he's had in a while. Until you get some starting point, it'll be difficult.
As for what you want the domain for, I would just say that you were considering a project and thought the domain would be a good fit. There are other domains you're looking at including another you have registered, but, you thought this domain had a slightly better fit.
Good luck with it.