Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow is meowing, very lethargic and has vomited.
I understand that he is on prescription food for urinary crystals but please check his litter box and see if there is any evidence of him urinating today. Unfortunately even with proper diet some cats will still form crystals and/or a mucous plug which leads to them becoming blocked, especially if they don't take enough fluids. Sometimes too by feeding a diet to prevent one type of crystal we predispose to forming a different type. I am most concerned that he has a urinary tract blockage with his symptoms.
A male cat's urethra is quite small in diameter. If inflammation is present or crystals form it can easily become blocked. When that happens urine builds up in the bladder, places back pressure on the kidneys which affects their ability to function, waste products build up in the blood, and electrolyte levels become unbalanced causing the cat to vomit and become quite lethargic.
If he is blocked this is a true medical emergency, he can die from the toxins and changes in electrolytes in his blood which can lead to fatal heart arrhythmias. He needs immediate veterinary care.
Here is a link if you wish more information about lower urinary tract disease: http://pets.webmd.com/cats/feline-lower-urinary-tract-disease
I know that he may not want to be touched but can you palpate his belly?
In the back part, near his rear legs/spine, is his bladder. If you can feel a balloon shaped structure, can be tangerine sized to softball size that is likely his full bladder. Some cats are so painful they won't even let you touch their belly, and so if he becomes aggressive with your trying to palpate his abdomen don't try.
I highly recommend that you have your fellow see a veterinarian right away this today.
Please let me know if you have any further questions.