Biblical assertions aside, I think this dad is not acting like much of a leader to his child.
My kids are 5 and 7. If I let them, they would wear Halloween costumes to school every day. Superheros, cartoon characters, you name it. Even when we get ready to go somewhere, I've had to tell them "That's not appropriate clothing for ..." fill in blank with destination or weather or whatever may dictate clothing choices.
There does come a time when children will need to make decisions and live with the outcome of those choices. Until that time, it is up to the parents to lead them. The most basic tenants of child-rearing, education, and whatever training begin with the 'correct' way, or in this case, societal norm. At the age of 5, this child should have a role model who recognizes that the dress-up and play clothes stay at home. He can 'express himself' when he has some life experience and a bit better awareness of consequences.