[quote=@tsukune]
Sure, the whole absolute zero thing sounds cool 
[/quote]
He specifically [i]can't[/i] reach the absolute zero as of now due to not being able to sustain his power for so long.

[quote=@tsukune]
a cap of how much heat he can absorb before he literally burns himself to ash
[/quote]
I thought there would be no cap at all? This is a part of his power, gradually absorbing all of the heat into himself, all of it. And then, letting out no energy before he uses it to fuel his buffing powers. This particular type of heat just doesn't hurt him due to [metahuman energy shenanigans], like some firebenders can spew fire from their mouth or create intense and most horrible heat in very close proximity to themselves without getting burned. Either that, or all of heat that he siphons is simply converted into the metahuman energy at a VERY uneven exchange rate.

[quote=@tsukune]
there's no way he won't get burned by a pyrokinetic spamming him with a barrage of lava-hot fireballs
[/quote]
He will be burned indeed, though the heat WILL be reduced to some degree and the damage will be further mitigated if he expends some of his energy towards the activation of a burst of super-durability.