I am sorry to hear that your fellow Dewey is acting painful and has vomited.
Please check his litter box and see if there is any evidence of him urinating today.
I am most concerned that he has a urinary tract blockage. 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 uncomfortable.
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
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.
I highly recommend that you have your fellow see a veterinarian right away this evening.
Please let me know if you have any further questions.