Speaking not as a Catholic, but from someone who has been a Nursery and Kindergarten Teacher.
This really isn’t that unusual, for girls or boys. Children who have often been sheltered at home in the sense they have had fewer opportunities to speak to other children outside the family unit actually tend to do this more.
Usually this is because at this age there is a kind of hierarchy in gender roles, and children are still learning who plays with what. They have no concept of biological difference, they usually work this out from mannerisms and negative feedback (i,.e: “You’re a wuss” or “Nice girls need to not get dirty”) A child who hasn’t seen other children like them only doing certain types of activity will not see any taboo in breaking gender roles. It’s performative in a sense.
The more time he spends in school, the less you see of this. By Kindergarten it’s usually entirely gone, because by that age boys know they will get beaten up by other boys if they do it, so even if they want to they know by then to hide it.