The only thing you can do at this point in time is to delete people one by one. We don't have an option to allow you to trim a branch of the tree.
(Our family tree software can do this, but it takes many steps to convert the tree, make the changes and import it back to your site, so I don't really recommend trying to use the software at this stage. However, if you would really like to know the steps I can tell you. Let me know.)
You can be a member in his family site and you can get Smart Matches with his tree, but we don't have a function that lets you join your two trees together so that if you click on his entry in the tree you can actually navigate to his tree. We're considering doing something like this in the future.