Hi there, did you check your gift box in the game? It would have been sent there for you to accept. If it's not there your son will have to contact FT support as it's his account that the FC was taken from.
To contact the support staff he needs to scroll down past the bottom of his game screen and the Friends Activity panel. On the left hand side, click on 'Report/Contact This App', then fill out a support ticket. IF it is agreed upon that you will be reimbursed, be patient, it may take time for them to get back to you. Responses can take anything form a few hours to 2 weeks, depending on the amount of support tickets already received.