I think it's more because his name has four syllabyla, than a difficulty pronouncing it. Besides, he didn't just say his name was Aximilli when they first met. He stated his entire name, which adds more to it.
I actually pronounced Tobias TOH-bee-as, before I heard the name on TV. Not the Animorphs show, but somewhere else. There they pronounced to Tuh-BYE-as.