Itunes won't transfer my music to new phone the way it used to.

I replaced my 4s with a used 4s that was wiped clean when I got it. I restored from cloud to my last backup and it's all working but it didn't include music (not sure if it should have). When I try to manually transfer music by connecting to my itunes on the computer (using usb connecter) it says I need to "erase and sync" because the phone is already associated with an itunes account and list's the same one I'm using I think, the same one used with the old phone. It does say something about that account being on "(my son's) computer" but I belive it's the same since the old phone had been my son's and we have always used the same itunes. Will I loose everything including all the apps and settings I just re-set up by letting it do this? Will it effect the library in Itunes at all? The phone didn't have any music on it at all after setting it up with the last backup restore. But I have noticed that at some point while trying to set it up and move music from itunes a few (and I mean very few) songs were added. I'm not sure when or why and I can't figure out why nothing will transfer now. I don't even seem to be able to transfer ring tones onto the new phone. I'm sorry this is so long and I'm sure basic but I have been looking all over for the answer or something about this "message" and haven't been able to find anything and wanted to include a. I've learned the hard way not to just let itunes take over and start erasing things without knowing what will happen!