More weight issues!

Wednesday I took Eli in to the pediatrician for several reasons.

The weird bowel movements were no big deal, the ped said it happens.
The low grade fever was attributed to teething and really the ped said that he wasn't even THAT feverish.
No ear infection.
We did blood work for the weird rash because it looks like petechiae, but the ped just said to keep an eye on it.
The blood work showed some "viral cells", so he might be getting over a small infection.

OK. That's fine. All answers that I suspected....

HOWEVER.... Eli has NOT gained any weight since the last time we went in. He weighs the same as he did almost TWO months ago. I know the nurse weighed him correctly, I watched. He is still only 16 lbs 14 oz. He dropped from the 75%ile to the 50%ile to the 25%ile and is now in the 10%ile. How is that possible?

Eli eats three large meals and snacks throughout the day in addition to breastfeeding. He SHOULD be gaining. The doctor seemed a little bit concerned, but brushed it off as a fast metabolism and being an active baby. He said he usually sees this drop off in older babies, but then it levels out.
In my mind, that doesn't really justify it though. There are plenty of 8 month olds who are just as active if not AS active as Eli... they still gain weight and chunk up. I'm just paranoid now that it is something else, some underlying condition.

I am strongly considering switching to formula so that I can know how much he is getting. What if I am not producing enough milk for him and I don't know it??

We talked about increasing his solids, but I am wary about this. I informed the ped of how much he already eats and told him that I DO NOT let him eat to his fill. I have always stopped him from eating. He has never REFUSED food. The ped said to let him eat as much as he wants. WHAT? Is that healthy? He is already eating SO much!
We tried this Wednesday night and this is what he ate for dinner:

He nursed for 10 minutes beforehand
Ate a whole Gerber Tray of Ravioli
4 Chicken Nuggets
A cup of fruit
Potato and beef stew (about 4-5 ounces)
A piece of bread
Juice
Then nursed for 10 minutes afterward
This was JUST dinner!!! I don't know where he puts it.

I just feel so lost on this. Like I am failing my child. He should be gaining and getting chunky, but he isn't. He is skinny. Babies shouldn't BE skinny.... I just feel like maybe I am doing something wrong. Even though I know that's not logical.

What if something bigger is wrong? What if he has some kind of condition or disorder or something that we haven't looked into??

I feel like I am going nuts! It's so hard when it's someone you love more than anything in the world and you think there might be something wrong. I never knew that my heart could be so full of worry and my mind could think the worst just by having a baby!!