Baby Bunnie Emergency! Help!!!?

I have a baby bunnie that i got 2 weeks ago (My third). And they told us that he was about 6-8 weeks, but he was so tiny , we didnt think so. But he hasnt grown at all since we got him, so now we think he was the runt. Anyways Jack (The Bunnie) Cant eat anything hay based or alphalpha (Sorry if i spelt that wrong), Without getting diarrhea. Today my mom gave Jack a little peice of lettuce, And he now has diarrhea again. Except its the worst one ive seen. Its covering him. I did some research and found out that diarrhea can kill bunnies?!?! Im extremely worried about Jack! Please help! I dont want my baby boy dying!