It’s entirely depends why he has to felt this phobia.
I definitely would not force him to eat it, but teaching him to touch it and accept bring near them is good.
When I was quite young around four I suddenly, seemingly overnight, developed an intense hatred of bananas, my mum said it wasn’t that I just decided I didn’t like them, it was like I suddenly had an visceral hatred of them,
I would react to the sight or smell of them.
Even banana flavoured things repulsed me and I couldn’t be near anyone eating them.
I think everyone thought I was just being a drama queen and I was scolded for acting silly around them but allowed not to eat them.
A few years later in my early teens I had a smoothie with banana in it, it wasn’t listed in the ingredients so I didn’t know, within only a few minutes of having a couple of sips I started vomiting and itching.
It turns out I’m actually allergic to bananas.
So the intense hatred was most likely my body’s way of telling me to avoid them.
I’m not saying your son is allergic to apples, but sometimes it’s not just psychological.
He should definitely learn to tolerate being near them though.