Build a strong and loving relationship with your wife. Spend quality time together, communicate openly, and show her that she is the most important person in your life.
Avoid being alone with him as much as possible. If he tries to approach you, make sure there are other people around. For example, if he invites you to his place, say that you can't because your parents are waiting for you or you have other plans.
Tell your parents or guardians about what happened. They can talk to the boy's parents. Also, avoid being alone with him in the future. If you see him coming near you, go to a public place or be around other people.
To prevent this from happening again, first, talk to your wife and see if there are certain stories she doesn't want shared at all. Then, communicate with your neighbor. You can say something like 'Hey, we really value our family's privacy, so if we share something with you, it's not for spreading around.' And be more selective in the future when choosing what to tell your neighbor about your wife.
You can install better window blinds or curtains. This simple measure can greatly enhance privacy. Also, if you have a backyard, you could plant some taller bushes or build a fence.