David Foster, a 72-year-old Grammy winner, has justified his 35-year age difference from his 37-year-old wife Katharine McPhee. Foster spoke openly about his new role as a father to PEOPLE after selling out his event An Intimate Evening with David Foster at Wynn Las Vegas’ Encore Theater on Jan. 21. “It’s cliche to say ‘Now I have more time,’ but I do: I have more time,” he explained. “Without regard for any of my other children, whom I adore and cherish just as much as my son.” “He plays Mozart and Bach flawlessly,” Foster jokes of his infant son, considering the prospect that his son might i…