Will this mess up my iPhone?

I have an iPhone 5c and my boyfriend just got an iPhone 4 and he wanted to use my iTunes stuff on his phone and I have it on my phone and iPod would he be able to use it for his phone as well or would that mess up my phone?

He'll be able to use it just fine, I have all my data from iTunes saved on my iPhone and iPod, and then my brother shares the same iTunes account for his ipad and he has his own separate music, apps, etc. It's just that once in a while, if he downloads an app, that app will install itself on one of my devices through iCloud, but it's not a big deal you just delete the app

It works fine but it can cause some issues with your account. I would recommend only sharing accounts with family members.