Cat anxiety

Question
I had a elderly cat and a young cat just a few years old, then adopted a feral kitten.  The feral kitten brought fleas which caused an infestation very quickly.  The elderly cat was allergic to the bites and became so anemic and weak that he stopped eating so didn't get his antibiotic for the rash.  I reluctantly had him put to sleep.  The young cat has now become very fearful.  He is afraid of the floor because he sensed fleas there (he has none now) and will not move from room to room except in great haste.  He has spend whole days in the bathroom so he doesn't have to leave the room.  He may also be intimidated by the extra-frisky kitten, who would like to play with him and won't take the hint.  What can I do for the fearful cat?

Answer
I don't think he's afraid of the floor due to sensing fleas, I think it's completely due to the feral kitten. Do they eat/play/sleep or anything together? Does the kitten try to play and if so, how does the younger cat respond? How old is the feral kitten?

Have you taken him the vet to make sure he's not hiding because he doesn't feel good? Is he still eating,drinking and using the litter box?

One thing to start with is giving the frisky feral kitten something else to do instead of bothering the other cat and that's using a noisy toy/object and tossing across the room opposite of where they are. How many times a day can you commit a small amount of time to implement strictly playtime and strictly calm time? And don't forget to dedicated time to the fearful cat as well as we don't want him thinking he's forgot about.

Something you can try that can show him he and the kitten can live in harmony is using my food mediator trick. Give a dish to each cat filled with mouth-watering food and place them as far apart as possible. Once both cats are eating in peace, gradually move the dishes closer together until they are side by side or one cat is eating out of the others' plate.

In the meantime until peace can be found, I would make sure the fearful cat has his own space with food, water, litter box and a bed.

Keep me posted as this can turn into a trial and error situation.