FTT and weight loss

I am going to call his ped tomorrow to set up an appt and/or call his GI also. I weighed him tonight with footie pajamas and a diaper on and I think he lost 3 pounds since last month. I know he's had a massive regression on food since we went to his aug comm appt but I didn't think he'd be losing a pound a week.
Has anyone else been through this? What did the dr. Have you do? I think they will tell me to keep doing what we're doing, but this can't be healthy.