Keep him away from TV, computers and phones, unless you can regulate what he sees.

Be aware of what he's being taught in school, and join with other parents in protest if the school tries to push gender ideology.

Be aware of his friend group and their parents.

There's a book by UK author Rachel Rooney (sp?) about accepting yourself. Geared to young children to innoculate them against gender ideology.

Constantly send the message that he is who he is and he's wonderful. Let him chose his clothes; if he chooses girl clothes be fine with it but remind him that he's a biological boy who can like what he likes without shame.

When I was at university, decades ago, I worked in a pre-school which was kind of a lab as well in that experts were observing the kids. We let the kids chose whatever they wanted out of the dress up box. We had one kid, around 4, who was convinced he was a girl for a whole year, and dressed like one, then went back to dressing like a boy. No one batted an eye at his choices either way, and that helped him find himself.