It's 100% related to no oxygen circulation, so my best diagnose would be cyanosis. Which usually affects the chest and around the neck, making the skin color turn blue/purple-ish. I'm surprised it didn't cause inflammation in his lips as well.
I don‘t think so, cause the livid colour is located on the chin too. The colour looks more than a hämatoma. The skin seems healthy and the typical white triangel doesn‘t appear. The lips would be livid not the tongue only