how to help my transgender friend

My friend Alex (birth name Abby) just came out as transgender. He was crying and scared so I cracked a joke and said "not surprised. my queer dar was going off, PEW PEW" and made finger guns at him. I'm usually pretty good with pronouns, but I want to know how to make him feel more comfortable in his boy clothing, as he goes to a catholic school... sadly in this religious school the sexual education is actually just "gays are bad, you must send them away to get treated."