Messages shows message under someone else's phone number

When my nephew sends me a text message, it comes up on my iPhone as a message from someone I work with. If I respond to the text, it will then appear under his name. Sometimes it will appear under both. This occurs every time, posting to the same wrong phone number and then eventually correcting itself. Both my nephew and co-worker have iPhones. I am using iOS 8.3. BTW, this issue doesn't exist when messaging on my MBP.

Check the settings on your iPhone: Make sure your nephew's name and phone number are listed correctly in contacts; the same for your co-worker. One possible way their names and/or numbers could be interchanged is if they're listed that way in contacts. If your contacts app on the Mac show the correct names and numbers, make sure you're syncing the contacts from your Mac to your iPhone. Of course if they're listed incorrectly on your Mac and you're syncing to the phone, they'll appear the same incorrect way on the phone.