By default, whenever you comment on a student's submission, any new comments (both from your student, and any other commenters) will be forwarded to you via an email notification.
If you'd like to disable his notification, you can simply:
1) Go into your Designlab mentor dashboard
2) Go to My Profile
3) Click on Notification Settings
4) Uncheck the Submissions Comments box
Please note that this will turn off ALL submission comment notifications, as there is currently no way to filter those.
If you have any troubles at all, feel free to get in touch with us at email@example.com.