Arrogant to me would mean, somewhat overestimating abilities, could try harder, attitude/commitment is a bit 50/50.
Its not really helpful to a child because they are unlikely to be able to break it down like that, and would just internalise it as a negative belief. I am sure that many posters had or know people who had comments that stuck from teachers.
So if it were me (parent) I'd just say there were these postive things x,y,z and she/I would also like to see you pushing yourself further in class and what could he do/what would help him do that.
If they don't have ideas make suggestions, you are good at x, so why don't you xx. Whatever it is...answering more questions, volunteering in class, asking the teacher for help, sit with different children. Maybe agree 3 goals. Thats where the teacher could help but its how you phrase the question...what would you like to see him doing more of.
I suppose its hard as the parent as you think well what am I doing wrong and you are not there to see.
I don't think that matters though, he probably just needs a bit of shining encouragement from you to reach his best.