how do I erge my son ipod with ny computer?

I have my own itunes account. And I notice I can sign in and out of each id. But of course only my apps..books...music... are there. The thing is the computer I used to synce my stones ipod to is gone. So, I need to synce both now to mine. It is telling me if I synce the apps it will replace what is on the ipod with what is in my itunes library. I dont want that I want it to put what is on the ipod in the library? Sor of syncing it to my library? If I do that and allow the synce to happen will his apps be gone? I am not sure how I will sync his ipod if I can not merge the apps somehow? I mean I am signing in and out of the proper ids.

ANy ideas?