Thank you for sharing your story about little Jimmy, you are certainly very dedicated owners!
Tail pull injuries are nasty and the resultant damage to nerves controlling urination can be devastating. With time, affected cats can improve or even regain the ability to empty their bladder but manual bladder expression is required in the meantime. In my experience most owners of these cats successfully learn to express their cats bladder so you're certainly not alone!
I can understand how important Jimmy is to your son and family and if he is otherwise happy and healthy according to your vet then their is certainly no reason to consider euthanasia.
In regards to your vacation, I would suggest calling Jimmy's veterinary clinic and inquiring if they provide medical boarding for their patients. Many veterinary clinics are happy to board animals that have special medical needs such as Jimmy. This would allow him to remain safe and comfortable while you're away and if there are any problems he will be in the right place for them to be recognized and treated promptly.
I hope you all have a lovely holiday!