Table Tennis .......
Tough age. It's pretty common that as kids begin to make the jump to adulthood they have more of their own independent thoughts abut what's important to them. Don't know your kid, but it sounds like he's starting to wonder why he's spending this much time on the sport. Probably easier to teach physical skills to a motivated teenager than the other way around.
As hard as it is, ask your kid about his love for the game, and truly listen to his answers.
My kid works his butt off to develop, and I think he was the same age when he told me he had some concerns that if he didn't "make it" (whatever the definition of that is to a 13 year old) I would regret having put as much of my time and resources into his development. I assured him that this was something he needed to love, that if he ever wanted to walk away I'd be 100% supportive, that I love the time we spend together, and that the time and money spent was done willingly, without reservations, and without any strings. He's worked harder than ever, since.