Skinny Baby! What's a mom to do??

At Jack's 2 month checkup he weighed 13 lbs 7 oz, this was a great gain over his birth weight of 8 lbs, and it looked like he was going to be an 80th percentile baby! However at his 4 month checkup two weeks ago his weight was unchanged. The pediatrician told me not to worry about it because sometimes babies grow on a stair step pattern rather than a steady curve. Of course, as his mom, worrying about it is exactly what I did. Over the last two weeks I have continued to notice that his clothes are loose and he seems to be getting skinnier! I called the pediatrician yesterday and told him that I believed Jack was losing weight so he told me to bring him in. Sure enough he had lost an ounce since his checkup 2 weeks ago. I know that is not much, but babies should be gaining not losing! Despite his lack of weight gain, he has had no problem growing lengthwise. He was 24" at his 2 month checkup, 26" at his 4 month checkup and yesterday he was up to 26 3/4". All that growing and no weight gain makes a skinny baby!! Since Jack has been an exclusively breastfed baby to this point, this leads me to believe that my milk supply is not adequate for him, which is devastating for me because I have tried so hard to prevent that from happening. I met with the lactation consultant yesterday and we discussed what I could do to try and increase my milk supply...so I am taking Fenugreek, I have rented a hospital pump to try and get every extra ounce I can and we are back to middle of the night feeds. I don't mind doing any of this, I just hope it works, I want to see my sweet happy boy put on some weight!! Since he is 4 1/2 months old the pediatrician also suggested that I try cereal. We tried last night, the pictures below say it all...

So we will see how the next few weeks go...