Seeing that Harry can react to things moving as fast as his fire bolt, which has a speed of 150 mph or around 67 m/s, he should be decently above Drogon in reactions.
I’ll vote for Harry here, his shield spells could block many harmful attacks, and from what I’ve seen his shielding can block 3.14 tons of tnt, so he’s faster in reactions and can easily make shielding for himself if that goes south. And his shields are higher then both the dragons KE and the fire breaths AP.
Now that we’re done with defensive options, we can look at offensive ones. Firstly Harry has a long list of potential spells that can help him in small ways, like the spell Muffiato, which will fill the dragon’s ears preventing hearing, or Langlock, which glues the dragons tongue to its mouth, which will make the fire breath harder to use, and stupefy for momentary stun.
For more direct options we have sectumsempra a spell which will create many gashes on the dragon when used, or the apply names shooting spell, which to the surprise of everyone, shoots an attack. Finally for an option that can be used if truly needed we have Depulso which can potentially remove Drogon from the field enough for a win (I believe the timeframe is one week) as the open field isn’t specified by given Drogon’s speed of “faster then horses” it will likely take a long time if sent somewhere very far.
And there are even more Defodio will gouge out large chunks, enfrigo which will explode things, and entomorphis, which if successful, will turn Drogon into a bug. While the dragon does have resistance to magic, making certain spells harder, I’m sure that not all of these options will be cancelled out, especially the simple ones like Sectumsempra and Defodio