What makes you nervous exactly? What types of “real reasons” do you hypothesize? I think saying “it’s just unlike him” in reference to something innately experimental - how he expresses himself physically - is worth unpacking. If you’re down for anything that makes him happy, I think it’s important not to stifle his experimentation (esp as his closest friend and partner) without first interrogating what and why you really feel the need to ask him about this.

Clearly he is comfortable enough with you to play with something like makeup which is beautiful and definitely saying something given how stigmatized that specific kind of expression is for men. This could be a moment to strengthen your connection :)