Billionaire Elon Musk's four-year-old son, X Æ A-Xii, joined him on stage at a pre-inauguration rally for US President-elect Donald Trump on Sunday. "Sorry, little X just followed me on the stage here. He is a very enthusiastic supporter, as you can see," Musk said. "What matters going forward is to actually make significant changes," the 53-year-old further stated.