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If he has an obstruction in the urinary tract that is an emergency. It doesn't sound like that is the case. If your vet said the bladder was empty and he is not painful when you touch the area it is most likely not completely obstructed anywhere.
I would be suspicious of either a urinary tract infection, urinary stones, or feline lower urinary tract disease (FLUTD).
I would go to the ER if:
1. He cries out in pain
2. He strains at the litterbox with NOTHING coming out.
3. You feel his belly and can feel a firm orange sized mass which is a distended bladder and it seems painful.
Otherwise, I would continue the antibiotics and encourage water intake and go in for a recheck on Monday.
Let me know if you have more questions.