He is recognized as a saint in Eastern Catholic churches. That makes him a saint for all Catholics. Latin Rite Catholics do not exclude saints venerated by Eastern Catholics, regardless of what Taylor Marshall or whoever says (Taylor seems to be the one preaching the most strenuously on the web about him not being a saint).
It’s true that his mother St. Helena (who, among other things, found the True Cross) is the more revered saint in the Western Church. However, St. Constantine is still a saint.
Furthermore, even if someone objected to you choosing St. Constantine the emperor, there is at least one other saint named Constantine in the Roman Martyrology, so no one can argue with you about choosing the name.