I am sorry to hear you are worried about your son's dog. My name is Rebecca. I am a veterinarian and will do my best to help.
How long has he had trouble walking; is this a recent change? How long since he ate?
If you check his gums, are they pink in color, or do they look pale?
He will become dehydrated if he goes too long without eating and drinking. He may not be urinating and defecating because he is not eating or drinking, but it is not a good sign if he is not urinating. He could be in kidney failure, or very dehydrated. You can check him for dehydration by gently pinching skin over his shoulders and releasing; the skin should go back into position in less than one second. If it takes longer, he is dehydrated.
If his gums look pale, this could be serious.
He needs to be examined, his temperature taken, and given supplemental fluids if this continues. Try to get him to take broth, Pedialyte or Gatorade, or to lick ice chips to try to get fluids into him. If you can, take his temperature. Normal is 101 to 102. If he is over 103 he has a fever; if he is under 100 he is probably dehydrated and may need supplemental fluids by IV or under his skin.
Let me know what other questions I can answer.