I would have the vet look at him for a kidney or bladder infection.
If he has an infection, one day you will be riding and he will go down with you
That happened with my neighbor and his 3 yo TWH
If the horse doesn't have either, I would also ask to have him checked for metabolic issues.
Even though he's awfully young for them, excessive urination also goes hand-in-hand with them. Doubtfull but still, there's always that odd chance
It isn't something I would brush off and at least talk to the vet over the phone