There are several options. As mentioned he could use iTunes Match.
But also you don't have to keep your entire iTunes music library on your iPhone. If you use sync I believe there is a sync check box in iTunes for every song. If the box is not checked the song will not sync.
Also, you can check the box in iTunes when the phone is connected to "Manually manage music". With this option checked it overrides syncing and you can just drag and drop the music in iTunes onto your phone that you want on the phone.
I have a large library and have always used manually manage music so I can just drag and drop what I want on my iPod and iOS devices.
Personally I like using iTunes Match just because I don't have to connect the iOS device to the computer to manage my library on the iOS device and also being able to have access to your entire library on multiple computers. You don't have to stream, you can still download select items to your iOS device and other computers. I usually download stuff I want when away from wifi and turn use cellular data off so I don't use my cellular data when I'm away from wifi like in my car.
However, iTunes Match does cost a fee of $25 a year (if I'm remembering correctly). But you also get iTunes Radio commercial free which is nice.
A couple of other things about iTunes Match. I had a lot of trouble with it with it introduced with iOS 7. With 7.1 it seems to be working ok at this point. But apple messed up deleting items on iOS 7 by Artist and Album. It's no longer an option. You have to delete song by song which is tedious. Also, in iOS 7 if for any reason iTunes Match gets toggled off on the iOS device all music downloaded by iTunes Match on your iOS device gets deleted off the device which is a tremendous hassle for me who wants downloaded items to stay on the device. So with iOS 7 it's been a struggle to keep my downloaded items on my device while using iTunes Match. Prior to iOS 7 iTunes Match could be turned off without deleting currently downloaded items. Apple seriously needs to reconsider this as it appears they have done this by design. Also, it is not possible to use drag and drop in iTunes when manually manage music is turned on when using iTunes Match on the iOS device. This has always been bugged me about iTunes Match.