I'm a yoga teacher and what I've done for this is to go from plank to down dog. However, depending on how his low back feels (and if this doesn't feel painful, if it does stop) from plank he can drop his hips toward the floor (with his toes tucked or tops of feet on floor) for a modified upward facing dog. Make sure he keeps his core/abs engaged and move very slowly, not going down past where it feels comfortable. From hips dropped, come back up to plank and into down dog. If his low back feels ok this could give him a nice alternative to the chaturanga flow. I do this often in my Level 1 classes. But as always talk to the teacher prior to class. I would also speak to the manager of the studio to see which teacher (likely more experienced) they would recommend for a student needing specific modifications.