Our 18yo dependent child filed his taxes, checked that no one else could claim him & now refuses to do the amendment just to spite us. What happens next for him & for us?

He did live with us 10 months of the year and was completely supported by us, but thinks he knows it all now and is doing everything he can to make our lives more difficult (hence why he moved out). But he refuses to do the amended return because he thinks that’ll be doing us a favor. What now?