Is the husband's saying, "I love my mother more than my Lord and His Messenger," considered an act of disbelief, knowing God's verse of warning of war against those who love others more than Him?

If "he honors his mother more" means greeting her, then there is no problem with that. However, if it means that he loves his mother more than Allah and His Messenger, then this is a grave danger. For loving someone other than Allah as much as loving Him is Shirk (polytheism), and loving someone other than the Prophet (peace be upon him) more than him is a deficiency in faith. Allah Almighty said: "And among the people are those who take others as equals to Allah, loving them as they should love Allah," and the Prophet (peace be upon him) said: "None of you believes until I am more beloved to him than his father, his child, and all of mankind." It is obligatory upon anyone who has fallen into this to repent to Allah.