2nd ipod on an existing computer

I have a video ipod. My son just got a nano. I use a credit card to buy songs and I have given him a $50 gift card to download songs (he is 12).
I have tried everything to add his ipod to my computer- he does not have a computer of his own. when I tried to activate an account of his own, it would not allow me to get beyong the credit card info part ( as I said he is 12 with no credit card- just gift cards)- it keeps saying I have left info out (the credit card info).- this was on my laptop.
I tried to download the songs he likes and transfer them to his ipod, but it won't let me do it under my account- how do I accomplish this.
both he and I are getting quite frustrated.
dell   Windows XP   one computer with vista


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