The rules say this about torches:
Torch. A torch burns for 1 hour, providing bright light in a radius of 20 feet and dim light for an additional 20 feet. If you make a melee attack with a flaming torch and hit, it inflicts 1 fire damage.
Light spells have this description:
You touch an object that does not measure more than 10 feet in any dimension. Until the spell ends, the object throws bright light within 20 feet and dim light for an additional 20 feet. The light can be colored as you want. Completely covering the object with something opaque blocks the light. The spell ends if you cast it again or discard it as an action.
If you point to an object held or used by a hostile creature, that
The creature must succeed on a Dexterity saving throw to avoid the spell.
The rules only cover these different levels of light: sunlight, bright light, dim light, darkness and magical darkness. There are no rules that indicate that you can create additional bright light by stacking the light sources and blinding a creature with it.
That said, like DM, it's up to you to allow or reject the things that players try. If the dragon had been in the dark for some time, it would be reasonable for a sudden bright light to be painful for him.