Simply living next to your neighbor doesn't allow you any specific rights to buying his property. At the same time, you can obtain what you are looking for rather easily. If he is thinking of listing the home for sale, and you know for a fact that he is selling the home, simply approach him and negotiate and offer to purchase the real estate. You can sign a purchase agreement and go from there. A blank purchase agreement is readily available either on the internet or any office supply store.

If he may be selling his house at some point in the future, but is not sure, you can negotiate an option to purchase with him. This is a little more complicated, because all you are doing in purchasing the option to buy his house should he ever sell. This will essentially allow you to buy the home at a predetermined price should he choose to sell. There are a number of variables that go into this. There may be a time limit on the option, or it may be indefinite. There may be a set pre-determined price for the purchase, or a graduating price allowing for potential appreciation. The amount of money paid for this right is also completely negotiable between you and the potential seller. If this is the route you need to take, you should definitely speak with a Real Estate attorney to obtain an agreement, and negotiate the finer points of that agreement.

Go over and make a resonable offer!

Option, as you pay a fee to have first right of refusal?

Or just "I want to be first in line when he puts home up for sale."

Honestly, if you know he is going to sell the home and you have a genuine ineterst in buying it.  Here's what you need to do:

1) Find out anticipated asking price.

2)Get a solid pre-approval from a reputable Mortagage Broker in your area.  A Mortgage Broker, because that will give you the most options for financing.

3)Make the offer before the home is put on the "open" market.  What I mean by this, is either deal directly with the seller or work with your trusted Realtor when putting the offer in, before ads go out or even a "for sale sign" is planted in the lawn out front!

Now go buy it.