As far as I know, diabetic is inherited genetically but crossing gender. A dad will only pass it to his daughter and a mom will only pass it to her son. Nowadays, even someone without any genetic inheritance can still from diabetic if he/she doesn't take care of his/her daily sugar intake and doesn't perform a healthy lifestyle -exercise, balanced diet. Even if you're boy is still a little plump, you can help him to be healthy and avoid diabetic by giving him the balanced diet he needs and encourage him to exercise (play sport, swim, and such). I wish you and your family to be healthy always! :)
Yes, there are strong chances that your son can have diabetes as diabetes is acquired genetically too. Please keep a check on his weight and inculcate habits of healthy lifestyle in him. Encourage him to play any sport. Stop giving him junk. Don't store junk food in the fridge. Its better to be aware from early on. http://www.diabetes.co.uk/diabetes-and-genetics.html