回复：他可以对美国军方说不吗?

He will be called a “deserter”, who is subject to military court martial, of course unless the future Presidents (don’t dream this one will do it) gives him an amnesty or pardon. Please just simply forget about coming back to U.S. legally. The above applies to American citizens. I’m not sure what U.S. Government would do with your brother-in-law, but I’m sure he would not receive more generous treatment.