Aiden almost 9 months

Yesterday I took Aiden to the Doctor for his well baby. When the nurse weighed him my jaw hit the floor-14 lbs???? At 5 months he was 17 lbs! What is going on? Why did he lose 3lbs. ? The doctor wants me to get his blood drawn to test all sorts of possible causes like thyroid, celiac, metabolic disorders etc. I haven't decided exactly what or when to do it but I am considering working for a month to get his weight up & then see. I am just baffled though. The other 4 kids were all way off the charts for weight & height. I worry about his brain development. He just barely started eating more than a couple bites of cereal or veggies when we feed him so now at least he's interested. He still nurses every 3 hours but I was told I may not be producing high calorie breast milk. It sure feels like he's getting enough so then I wonder if he really does have some kind of disorder. Aiden is such a sweet & patient baby & to be honest I love having him little longer! But not this little!