Can a baby move from vertex to breech position after 38 weeks? he is in vertex position from the very beginning.currently he is at -1 station

Yes: It is possible for the fetal presentation to change at term but it is unlikely especially with the vertex at -1 station. If you are worried that the baby has flipped discuss your concern with your obstetrician at your next visit.

Presentation: A couple different things to consider when the presentation changes. The first is that your pelvis is too small or baby is too big for your pelvis to become engaged allowing baby to change position because it is high in the pelvis. This can be seen with an unstable lie (lie is the position breech, vertex, transverse). The other issue would be in multips with a small baby that can easily flip.

Maybe/unlikely: You will know the final position when you go into labor & changes do occur. However, it is much more likely earlier in pregnancy when the amniotic fluid/baby ratio is higher & there is more room for them to maneuver. If He has maintained a steady position for 38 wks, it is likely to stay that way. However, I wouldn't make any bets on it.