Hi mysak, I am sorry to hear of the problems you are having with your cat.
It sounds as though you have a very unhappy cat! If the vet is certain there are no health issues with him, then it must be the environment that is affecting him.
Bengals are a very intelligent breed of cat that need a lot of stimulation and entertainment.
They also need a lot of companionship, so if you are not at home with him most of the day, it would have been a good idea to have gotten a cat companion for him at the same time you got him. But bear in mind it may not be easy now to bring another cat in to the house, as your Bengal cat may not accept it. But it would still be worth considering, if you can find a young cat that would get on with him.....perhaps another Bengal.
Bengals are a breed that need a lot of exercise, and if yours is an indoor cat, you need to provide him with plenty of challenging activities, and also you need to play with him interactively for an hour or more every day without fail.
You should keep introducing new challenges for him, to prevent him getting bored with the same old things. If your cat does not have enough to occupy him he will become miserable and bored, and become withdrawn. I feel this is maybe what has happened, that he has become withdrawn through lack of stimulus.
Buy some fishing rod type toys such as Da Bird, and a laser light, and little ping pong balls for him to chase. These are all toys you can use to play with him, without invading his personal space. Kitty can decide to come to you when he is ready.
As Bengals are cats who love to climb and run, I would seriously recommend you get some of the following items for him, if he is an indoor cat.
either
Cat Exercise Wheels - The Maclaw Wheel - this has been designed especially for Bengal cats.
or
Cat exercise wheel for exercising your cat
and
Cat wall - the long lasting cat tree
The Curvynest is a cat climbing tower
As well as mental stimulation, with toys and games, your cat needs a lot of gentle encouragement to be affectionate to you again. He sounds as though he has become wary and mistrustful of you -- can you think of any reason why this should be? This kind of behaviour can happen if a cat is left alone for long periods, or has been reprimanded a lot.
Try the following:- sit on the floor so you are down at his level. Have a length of string and draw it slowly towards you across the floor so he chases it. When he gets near you reward him with his favourite cat treats. Then gradually bring the string close up to you, and on to your lap. Continue to reward him with treats whenever he gets close to you. Do this every day for a few weeks until he is confident being close to you again.
Make sure meal times are occasions for him to really look forward to. Feed a variety of high quality high meat protein wet food. No dry food. If you are out all day, leave meals of wet food in auto-feeders. Buy the auto-feeders with removable freezer packs you can put underneath the dishes to keep the food fresh.
I think it is going to take time and patience to win back the affection and trust of your cat, but is surely worth doing, so your kitty is happy with his life again. I see no reason why things should not improve a lot if you follow the suggestions I have made. Best of luck!
