He’ll have to announce his pick at the convention. That’s how it works, and he may not know, with certainty, who he’s running against until the Democrats hold their convention, in August.
There’s nothing compelling him to announce at the convention, but that is what is expected - and he won’t rock the boat without good reason. He will want to solidify his nomination and unify the party - selecting a running mate is part of that.
However. President Trump will be loathe to choose a running mate without knowing who is at the top of the Democrat ticket - he will delay as long as humanly possible if the Democrats have not decided whether it’s Biden, Harris or someone else.
Fortunately, the Democrats are desperate to unify their party. They will want to put an end to the post-debate chaos ASAP, so I doubt they will wait until their convention to nail down their ticket.