Simple It's a Charisma check using diplomacy to Convince the Dragon to move from indifferent to Friendly. Make the PC use insight to determine the characteristics of the creature. Give advantage if they play on the characteristics they get advantage if they go against it they get disadvantage otherwise it's a straight roll. If they move the dragon to hostile (by failing by 10 or 5 or whatever it says) you fail permanently, if your check says you fail and the dragon stays indifferent you can't really use it as a mount either. This is permanent unless magic or serious action ( save it's life etc.) Then it is 250 days at 1gp per day to train the dragon to accept a rider. This then gives you a Dragon that only needs charisma checks made to convince it to take a serious risk and doesn't need a check to ride in combat.

• It's a Charisma check using Diplomacy to convince the Dragon to move from "Indifferent" to "Friendly".
• Make the PC use insight to determine the characteristics of the creature. Give advantage if they play on the characteristics; but if they go against it they get disadvantage; otherwise it's a straight roll.
• If they move the dragon to hostile (by failing by 10 or 5 or whatever it says) you fail permanently, if your check says you fail and the dragon stays indifferent you can't really use it as a mount either.

This is permanent unless magic or serious action (like saving it's life etc.) Then it is 250 days at 1gp per day to train the dragon to accept a rider. This then gives you a Dragon that only needs charisma checks made to convince it to take a serious risk and doesn't need a check to ride in combat.