Year old kittens won't eat

I have 2 nearly adult kittens in my household.  Both have been ravenous eaters all their young lives.  They typically eat 2.5 meals a day - two of canned food, a half meal of kibble.  

The boy, Zadok, stopped eating his usual amounts two days ago.  I have seen him eat some (around midnight last night, he ate half a can of wet food) but he refused his breakfast and dinner today.

Serendipity, his sister, has eaten more than he has, but she's also turning away from food.  

Zadok has a notoriously tricky tummy.  As a young kitten, he got explosive diarrhea once that had him crying before it came out in gouts.  Sometimes, he has wet and runny poop, which gets all matted in his tail and hind feet.  None of this has happened since last summer, so we thought he was over it.  He was taken to the vet and checked out when he had diarrhea, and a change of food seemed to do the trick.

But no matter what we offer him, he won't eat.  He is, however, drinking.  He's much lower in energy than usual, and wouldn't really even play with his favorite wand toy.  Sister has a little more energy, and jumped after the feather, but not with as much enthusiasm as usual.

The only change in our household is that we're fostering another feral cat.  She's 5 months old, she's completely isolated on our screened in front porch, and she's been checked for all illnesses including Feline leukemia.  She's been vaccinated and spayed.  He has smelled her once, then backed off.  His sister has not had any contact with the little girl.

I can take him to the vet if that's what needs to happen.  However, I've been down this road before with non-eating kitties.  Hundreds of dollars in tests later, it's inconclusive.  They pass whatever is in their guts and we never know why they stopped eating.

Tonight, if Zadok doesn't eat anything, we will do some force feeding.  But I hate the thought.

Ideas?

I'd be very concerned about two days of inappetence, and follow the rule of a vet visit if the cat isn't back to eating after one day. Even though the foster has been given a clean bill of health and there's been little or no contact between the cats, a household member may have transmitted something from her to the kittens that her mature immune system can deal with, while theirs can't.

Both cats not eating makes me wonder if it has something to do with the food. Maybe a bad batch, or maybe the bag of dry food got hotter or wetter than is ideal and went bad. Something contagious sounds entirely possible, too. Is the poop normal (or is there no poop, either)? Is the kitten on the same food and still eating it?

I actually have 8 cats in the house, and several feral cats out back.  I had 5 cats for about a decade, then the fostering we've done has resulted in some of the temporary young cats being permanent residents.  Zadok and Serendipity, the year old kittens, were rescued from my ravine after we trapped and spayed the mama cat.  She refused to nurse them, so the babies came inside and were bottle fed.  We had homes for all three of her litter, until we found out that the people adopting Serendipity were planning to declaw her.  Then Zadok's new family brought him back - I think because he had some poop issues after being moved to new food.

Castor is a legacy from the socializing event of the previous summer - 4 boys, all about 4 months old, all socialized and homed except him.  Now, he's ours.

None of the other cats show any problems with the food.  The kitten on the porch eats like a champ, and is making wonderful progress toward trusting people.  

Zadok ate part of his breakfast today - HOORAY!!!!  And now Dippity isn't eating much.  I think Z is constipated, but perhaps the laxatives he's been getting are starting to work.