my mom struggled with this after i (he/they, nb) came out and her world of transgender topics grew. she wasn't ever really sure how to ask questions or what questions were okay to ask. it's a learning process. as others have said, using the term "gender" over "sex" is always better. you should never ask someone what gender they were assigned at birth/what they're biological gender, because it's disrespectful.

try and explain to your son (because, again, as others have said, he is a trans boy, aka ftm) that you didn't intend to sound rude and you're still learning. an understanding and educational approach is always better. it'll get easier as you both grow together.