I'm sending this case history for a fellow LC who doesn't have lactnet.  Put your heads together and send us some ideas!

Case - 
3 month old boy - mother's 10th breastfed child (all but her 2nd BF well and doubled birth weight by six weeks of age. Mom was very sickafter 2nd pregnacy and lost her milk supply. Since then all babies have done well.)  Still occationally BFing a toddler.   
Birth weight 9lbs. 12 oz.  4415 gm
Weight at 3.5 weeks 10lbs.3 oz  4645gm  Weight has fluctuated slightly from this point on
Weight now 10 lbs 2 oz. 4640
creamatocrit shows 10% milk fat - equal to 30 calories/oz. This was a total sample, not hindmilk. 
Frequent urinations120-180 cc diapers - well hydrated, good turgor, soft font
1 stool every 5-8 days, thick, green, hard to pass, and very foul smelling
Repeated consecutive Pre and post feed weights show 3-5 oz intake,
feeds every 1-1.5 hours, ½ hour after feed shows pain and becomes dusky, will nurse for comfort then sleep short time. Feeds every four hours at night

Upper GI normal with some reflux - not projectile, just spit up.  Radiologist mentioned to mom that he saw slow gastric emptying, then he saw the reflux.  Gastroenterologist only addressed reflux.
Lower GI normal
Baby took reflux medicine Zantac. Propulsid, and Reglan no improvement, maybe worse
Have ruled out CF(sweat test) and Hirschprungs disease

Fecal fat - negative

BAby had surgery a week ago to remove gall stones.  Surgeon also repaired bilateral inguinal hernias, and removed a polyp on one testicle.  Infant was expected to pick up weight after surgery but has not.  

While baby was in surgery mom hand expressed 22 0z of milk.  supply is not a problem. 

Any ideas what could be causing weight loss?  What can I suggest to Pediatrician?

Connie Carlisle, RN, IBCLC