If my son eats a ton of junk food, how likely is he to get diabetes, he is 12yo?

Depends. Family history, ethnicity, bmi, activity, puberty, gender --- all play important roles besides junk food in causing type 2 diabetes at this age.
depends... If he becomes obese, the likelihood to develop type 2 diabetes is certainly high. The chance is even higher if the affected child belongs to ethnic minority or has first degree relative with type 2 diabetes. Besides, he should be encouraged to take enough dairy products and vitamin d for his bone health.
Avoid . No juice from now on if he want whole fruit is healthier and tastier.