Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Little Gray isn't as playful as usual and isn't eating well.
While your older cat's bullying could make him hide and perhaps be a little less playful in front of your older cat I would not expect his appetite to be affected. I think that this little one isn't feeling well.
And I suspect that your older cat is feeling that he can bully him because he isn't feeling well and isn't strong enough to fight back right now.
I am concerned about a consistent decrease in his appetite.
In a young cat a change in appetite and lethargy can be related to eating something he should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), a dietary allergy or sensitivity, toxin exposure, or a viral or bacterial infection.
Because he hasn't eaten normally in several days I am very concerned about him. If he isn't drinking well either he will quickly become dehydrated, and as his liver breaks down fats for energy to live he may develop a type of liver disease called hepatic lipidosis.
Ideally he would see a veterinarian today if this has been going on for more than 72 hours. Simple stomach upset should pass within 24 to 48 hours. They could examine him, run blood tests and possibly check radiographs and/or an ultrasound to evaluate him and know best how to treat him.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate him, both of which will make him feel better.
If you cannot have him seen for whatever reason today there are some things you can try at home.
At home to try and settle his stomach and improve his appetite you can give either:
1) Pepcid ac (famotidine) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and may help him feel less nauseous so that she will eat better. They are quite safe and can be used for several days if necessary.
You can use a medicine syringe to try and force water into him orally.
A couple of hours after giving the acid reducer so stimulate his appetite I recommend offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If he continues to not eat he should see his veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.