My 6 weeks baby has bruises onhis lower back. should i be concerned, we just travelled a 20 hour train ride he did well and all. he was always either in his doona stroller or in my lap or lying on a soft nursing pillow. he is g6pd deficient also?

The description is too vague to be sure. Many infants have bruise like birthmarks that may not become evident until older. These are typically over the back and bottom of the spine in an area covered by diapers. Only a visual exam can distinguish these from bruises from some event. I would discuss it with his doc at the 2 mo check or if it freaks you out arrange a visit.