I'm sorry your dad is still suffering.
Everything I've read, including this - https://my.clevelandclinic.org/health/treatments/17917-holmium-laser-enucleation-of-the-prostate-holep/procedure-details - mention that some symptoms, usually frequency, urgency, and waking at night may take several months to improve, but I can't find anything about burning and flow.
This - https://my.clevelandclinic.org/health/treatments/17917-holmium-laser-enucleation-of-the-prostate-holep/risks--benefits - says that there may be burning after for several weeks.
It talks about flow, but I'm not sure if it's the same flow your father was experiencing.
Your best plan would be to talk to his doctor about his specific symptoms. Don't hesitate to call the doctor with any questions. That's what they are there for, and they know your father's history, and can best answer the questions. Please let us know what the doctor says. I'd be interested to hear what happens.
Best to your father, and to you.