He had two interactions of note with Trump. One was on May 23 when he and several other officials briefed the president on Zelensky’s incoming administration:
That’s useful to Trump in that it provides a non-Biden reason he might have wanted to block the aid even before a hold was put on it — namely, hard feelings. Fast-forward to early September, after the hold on the aid had already been in effect for weeks, and Johnson calls Trump to ask what’s up:

The hard feelings about corruption related to 2016 lingered, as did his usual complaint that Europe isn’t pulling its weight. Then Johnson asked him straight out about a quid pro quo. Trump denied it emphatically:

That’s consistent with what Johnson told the Wall Street Journal last month. He also told the Journal in that same interview about a conversation he’d had with Gordon Sondland at around the same time. That account isn’t as perfectly consistent. Here’s what the Journal reported:
Mr. Johnson said he learned of the potential arrangement involving military aid through a phone call with Mr. Sondland that occurred the day before Mr. Johnson spoke to Mr. Trump. Under the arrangement, Mr. Johnson said Mr. Sondland told him, Ukraine would appoint a strong prosecutor general and move to “get to the bottom of what happened in 2016—if President Trump has that confidence, then he’ll release the military spending,” recounted Mr. Johnson.
“At that suggestion, I winced,” Mr. Johnson said. “My reaction was: Oh, God. I don’t want to see those two things combined.”
Here’s how Johnson remembered that call with Sondland in today’s letter:

The first account makes it sound like it was the linkage between the investigation Trump wanted and the military aid (“those two things combined”) that bugged Johnson. The second account makes it sound like the mere fact that the aid was being held up was what bothered him, whether or not it was “combined” with anything. But I take it Johnson would say that he meant the same thing in both cases, that he didn’t want anything impeding the release of the aid, not that he thought Trump had an improper motive in demanding the 2016 investigation.
All of this is useful to the White House in offering a non-impeachable narrative for Trump’s actions — that he was suspicious and resentful of Ukraine after the last election and didn’t trust them not to squander the money. And Johnson’s claim that Trump resented Europe for not contributing more to Ukraine’s defense is borne out in the transcript of Trump’s call with Zelensky on July 25, when he made that point to the Ukrainian. The problem for Johnson is that Trump also mentioned CrowdStrike and the Biden/Burisma matter specifically to Zelensky in the same call; the president obviously was keenly interested in those subjects, not just Ukrainian corruption generally, to have broached them personally with his counterpart. Likewise, Gordon Sondland somehow came to “presume” that Trump wanted Ukraine’s military aid made contingent upon their cooperation on Burisma and made that point to them personally, by his own admission, in early September. Assuming David Holmes is telling the truth, Sondland was convinced by the end of July that Trump didn’t care about anything happening in Ukraine apart from the investigations that might benefit him. All of that supports the quid pro quo theory that Johnson views as baseless. Sondland is set to testify a few days from now. Get ready.
One more bit from the letter, this time Johnson describing a meeting that he and Chris Murphy had with Zelensky on a visit to Ukraine in early September, after everyone involved knew that the aid had been delayed:

That’s nice, but we know from reporting as recently as today that the Ukrainians had been feeling pressure from the White House to investigate the Bidens as far back as May. Soon after the meeting with Johnson and Murphy, Zelensky made plans to formally announce the reopening of the Biden probe on Fareed Zakaria’s CNN show on September 13. (Ironically, the aid was released two days before that and the interview was canceled.) Maybe Zelensky decided it’d be risky to complain to Johnson about the delay in aid, the Biden pressure, or both, fearing that Johnson would carry that message back to Trump and Trump might react unpredictably. I … don’t think Zelensky’s going to testify, though, so we’ll have to wonder.