Hello my cat has been poorly since yesterday he is very lethargic, has trouble walking and climbing and only eats a tiny amount of food. He is only 3 years old.

Check his food for wheat gluten. If it has it in it get him to vet soon.

Is he an indoor/outdoor cat? If he spends any time outdoors at all, he needs to be tested for Feline Leukemia/FIV immediately. He also could have an abscessed fight wound smoldering somewhere on his body that needs to flushed, meds given due to fever and infection. Please see your vet soon.

Hello my cat has been poorly since yesterday he is very lethargic, has trouble walking and climbing and only eats a tiny amount of food. He is only 3 years old.
I would personally make sure he is drinikin fluids:) if he doesn't like being picked up or touched near the stomach area he might have some sort of infection and you might want to take him to the vet... (my cat was doing the same thing... but she is female and she had a milk gland infection)