Check out asmp.org. they have a whole section on their website about theft like this and your options. Stealing is stealing, right? If he's going to get paid or get jobs from your work under the false assumption they are posted by that IG account as their own... That's taking money out of your pocket and food out of your mouth. The professional photography community is small compared to many other professions, it can be very competitive.

it gets into some interesting but murky waters on the IG though. You basically give up your rights to the images you upload to their platform, but that has gone up for debate in court. But their are plenty of arguments to be made about your creative works and retain ownership.