There's various apps that can do this. Skype use to be the big one, but many others are getting in on it too. You'll have to install the same app on your computer and his phone and register accounts to do it, though. Facebook Messenger has the ability to do this, which may be an option already available to you since so many people use Facebook.
Facebook Messenger and Skype are the main ones I use. If you want a dedicated app, look for ones on the Play store that have a website, and have downloads for both Windows and Android