Slippery elm isn't really dosed by weight. I'd start with about 1/4 tsp twice a day mixed with wet food and increase up to 1/4 tsp 4x/day if you don't see any results.
Also, what is the kitten eating right now?
Probiotics certainly wouldn't hurt, especially if his diet has been changed recently.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler