4 year old being treated for Lyme and Lyme arthritis- doxy?

Hi! My 4 year old was discharged from the hospital yesterday. He’s being treated for Lyme and Lyme arthritis in his knee. They gave him a month long treatment of doxycycline 2x per day. I was talking to someone about it and they said it can cause his teeth to be permanently stained brown or gray. Is this true?? Not sure who to call. Discharge paperwork just gives the general hospital number.