Sressing over my foster cat

I have been fostering 2 brothers for about 7 weeks.  They were severely thin and infested with worms and fleas so bad they were in their eyes when they arrived.  From what I have ben told they were only fed on payday.  Anyway long story short, whilst hey have been having everything they could need the one brother has gained his weight back and is on the low side of normal.  His brother has continued to drop weight.  So far he's lost almost 400g.  He was already bones with fur, now he is dreadful.  He's had bloods done, tested for FIV/FeLV and spent day in vets having ultrasound and x-ray.  Apart from anaemia nothing has else been discovered.  His initial blood have not revealed anything gruesome and have been sent of to the laboratory for indepth analysis.  They come back on Tuesday.  He eats well and always has done since arriving.  The vet said he is a complete enigma.  I am fearful that I am going to lose him.

Does anyone have any ideas

Have you wormed them both for roundworms AND tapeworms?  With all those fleas, they are very likely to have tapeworms.  You might see the little rice grain size segments, but you might not.  You should use pyrantel pamoate for the roundworms and praziquantel for the tapes.  You may also want to try a higher calorie food, and adding kitten formula (such as PetAg's KMR) to their diet.  Other than that, they may just have different metabolisms just like siblings sometimes do, where one is chubby and the other is slim even though they eat the same thing.

I'm devastated to say last night he was put to sleep.  His white blood cells were extremely high in everything they could possibly be in.  Our vet said it was his bone marrow.  The only option was to open him up and have a look inside. But the chances of him surviving the anaesthetic was slim and even if he did, his body had little chance of recovering.  So the decision was made to let him go.  His brother is now being tested to see if he has it too.  I am heartbroken.

RIP Quest my little ginger Prince

I'm so sorry you had to let him go but at least he knew love and caring at the end of his short life, and that is a blessing.  It sounds like you are saying he had leukemia, but perhaps you just meant he had an infection with a very high white blood cell count.  I hope his brother doesn't have the same thing and will be praying to the Cat Goddess to watch over him.