unexpected change in child custody after K-1 Visa was approved - now what?

[mimi and sean 9]

My fiancee was approved for his K-1 visa from the Vancouver consulate in Oct. He is planning on entering the US April 1, 2012 (expiration on visa is April 12). On the paperwork we filed for the visa we listed his 2 children. During the interview we were asked if the children would be following him and we told them that neither one would. The person who collected our paperwork had my fiancee quickly write something up saying that the children would not be following. We were approved and have been preparing for his move to the US. Just yesterday my fiance was surprised by his youngest childs (child is 8) mother stating that she could no longer handle the child was having personal issues and she was going to sign over full custody to my fiance. We were shocked but of course we will gladly raise the child. The problem is what do we do now? How do we get him a visa? We are not sure if the child would be coming in April or finishing the school year out and coming in July. Would it be better if my fiance enters the country first and then we pursue bringing the child over or is there something we can do to have him added on now? We would appreciate any advice/help anyone has, thanks!