Okay, so here's what was ultimately decided.
Kastor can keep his tribe, but after so many generations, the bloodlines would have been so diluted that they weren't any different than regular humans. They wouldn't have had the powers of the kingdoms because of this. Kastor, however, because his mother was pure Thalassian, has the option to have water/ice magic and he would have a higher tolerance to the cold than others in his tribe would have.
He's basically like the people from Ventiaus and Ištarazil, who have equal tolerance for the weather in both Thalassia and Ignius Aēsma; but, this also means he'd be more vulnerable to the weather in Ignius Aēsma than his tribe mates would have been and has a higher chance of heatstroke. All because of his pure Thalassian mother, who also couldn't have gone to Ignius Aēsma as a nomad without some kind of protection from the heat.
Azura and I both agree that a tribe as old as his would have had more than a few treasures, so you have permission to give him an item called an a fire stone. It originally emits heat, allowing someone form Ignius Aēsma to be in Thalassia, but when it's enchanted, it can be reversed to absorb heat to allow someone from Thalassia to be within Ignius Aēsma or one of the other kingdoms. This wouldn't be a dire need for Kastor since he could simply keep himself hydrated and he'd be fine, but it is an option.
If you do go for the stone, it's worth noting that the stone he'd have would be red. A normal one is blue.
EDIT ::
Also, that meme was me the entire discussion with them. Not even kidding. Just give him a red face and an angry vein on the head.
rofl