Spit up after nursing

Hi ladies, 
My second baby boy was born 3 weeks early at 6.5 pounds and has jumped from 30th percentile to 60th by now. Looks like he growing at a good rate but his appetite is not growing at all. In fact it seems like his capacity to have longer feeds is dropping. He still nurses 3 hours apart and becomes uncomfortable after nursing 10-15 mins on one side. Many times he has this whitish curdly spit up (see photo)..
This is affecting my supply in the sense that think my supply is going down.
If I mix formula and breastmilk (1:3) he drinks about 3 on average.I feel like he isn't full but then he doesn't let me put him on the other breast and after a while babbles himself to sleep. 
Is this normal? The lactation consultant I saw said either he is overfeeding himself or is allergic to protein in breastmilk or has a bit of reflux going on. 
Please see pic and let me know if this has happened to your baby. What did you do?