Have you tried a kitten food for him? There are a couple on the food list that are higher in calories. Might be helpful with the weight issue. I've found that my kitties both like to have a variety of foods.
Jester's so picky and his previous reactions to changing his food (i.e. not eating at all) make me ultra-hesitant to try anything he hasn't previously "picked" on his own.
I tried some YA a couple of weekends ago and he chomped down like crazy on it, then later was begging for food but walking away from everything I gave him.
The episode last fall in which he stopped eating almost completely on his own for ten days was pretty terrifying for me and gave me an almost Pavlovian automatic response to any changes in his diet.
If I feed them the same thing several meals in a row they'll stop eating it, but if I keep things rotating they will allow a food to stay in rotation for a long time.
That's sort of what I do with Jester as well, although the rotation tends to be with two or three flavors. Turkey & giblets, chicken, and turkey & cheese - all Friskies shreds.
tbh, before dx, his favorite food was Friskies Indoor variety Chunky Chicken w/ turkey and rice. There was another flavor he liked of the same kind, but the only difference was that turkey was the main ingredient instead of chicken.
He ate that flavor of cat food for literally *years* and loved it. I'd still be giving it to him, but the carbs are in the mid-20s.
It's also possible he's just feeling a little
meh 
from the vet visit yesterday. Hopefully he'll eat better for you tonight.
I hope you're right. He's just scaring the crap out of me right now.....
btw, I finished entering the small amount of data from Jester's lab work and posted to the main board. Not much response yet, except that the numbers seem good.