Cant exactly say what could be the cause for his increased urination now.
But yes some medicines cause habituation.
Thing is Betheran or Bethanechol causes incresed urination .
So stopping it should actually cause difficulty in urination .
Whstever it is , using the drug for a long time may not be safe.
Next Steps
Better consult a paediatric neurologist or regualr neurologist for further treatment .