There are many causes to the problems you are describing, and almost all of them do involve pain. These problems include: disk disease, or another musculoskeletal injury (sprain, strain, etc), Lyme Disease, (or other painful joint disease spread by ticks), colitis, pancreatitis, urinary tract infection, and many, many, more. Even something simple like impacted anal glands could cause shaking and inappetance.
Any pet who is in pain should be evaluated ASAP to at least control the pain initially, than, once the pain is controlled, the cause of the pain can be identified by diagnostic procedures.
It is very difficult to adequately hydrate any dog by using a syringe to administer water. You would have to spend literally every waking hour syringing water into your dog's mouth in order to give him his daily water requirement. Your dog's vet can administer sub Q fluids to adequately hydrate him in a short period of time. You can be shown by the veterinary staff how to perform this procedure and should be able to do it at home, if necessary.
Your dog is still very young and has plenty of years ahead. Please take him to the vet!
please get ur dog to the vet asap!unwise to give ur dog aspririn or anything else unless u know his medical condition and if not advised by ur vet!