Q: 2 iphones on same itunes account

The recommended approach is for each person to have their own Apple ID and to then use Family Sharing. See the following links:

http://www.apple.com/icloud/family-sharing/

Family Sharing - Apple Support

Set up Family Sharing - Apple Support

He can do it from the phone without connecting to iTunes on a computer. He will lose access to all of his previous iTunes purchases (apps, music, etc) unless or until you set up Family Sharing as discussed in my previous post. Following are instructions on signing out of the old Apple ID and then back in with the new one: Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support

The following previous discussion discusses other potential loss of data and may help: If I change Apple ID will I lose all my contacts