A 5 pound dog would get 50 mg of amoxicillin every 12 hours. However, even though your dog is having similar signs as before, I still encourage you to have a urinalysis done. It is possible that he has crystals or blood that might indicate a bladder stone, which will require more than just antibiotics. Transitional cells can be found in the urine which could mean bladder cancer. And if he is urinating a lot and thus having accidents in the house, it could be due to an endocrine disease (such as diabetes).
So try the antibiotics if you have some, but if the symptoms don't improve in a couple days, I would take your dog to your regular vet.