These 2 underlined statements contradict each other. Also the first statement would get one or both of our returns hung up because they don't match!
You should say that no one else can claim him as a dependent then you claim him as a dependent with all rights.
He can still file a return just to get his missing stimulus with $0 income. The system is setup to allow him file but he must say that he is being claimed as a dependent by someone else.
