Need help with "upgrading" my I-130 app

[ValerieP]

Hope someone can shed some light on this subject for me. I have filed an I-130 for my husband who is a Canadian. I am a US citizen. The form has been approved and we are now in the NVC process. My question is.....my husbands ex has now decided that he can bring his 5 year old son to move with him into the states. We listed his son as his son on the original I-130 but how do I proceed with this? What additional forms do I need to file for his son to be able to join him when his visa is approved?

I want to file whatever is needed as soon as possible to try and avoid further delays.

Thanks in advance