**Core Concept**
Type 2 diabetes mellitus (T2DM) is a complex disorder influenced by genetic, environmental, and lifestyle factors. It is characterized by insulin resistance and impaired insulin secretion. The risk of developing T2DM increases with a family history of the disease, obesity, and age.

**Why the Correct Answer is Right**
Given the patient's young age, normal BMI, and lack of ketonuria, it is less likely that he has type 1 diabetes mellitus (T1DM), which typically presents with ketonuria and requires exogenous insulin for survival. The patient's family history of diabetes, particularly the fact that his father is the only son of his grandfather, suggests an autosomal dominant pattern of inheritance. This is consistent with a diagnosis of maturity-onset diabetes of the young (MODY), a subtype of T2DM that is caused by mutations in specific genes that regulate insulin secretion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Type 1 diabetes mellitus (T1DM) is unlikely in this patient due to his young age, normal BMI, and lack of ketonuria.
* **Option B:** Type 2 diabetes mellitus (T2DM) is possible but less likely given the patient's young age and family history suggestive of an autosomal dominant pattern of inheritance.
* **Option D:** Other forms of diabetes, such as gestational diabetes or secondary diabetes due to pancreatic disease, are not suggested by the patient's history and presentation.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a family history of diabetes, consider the possibility of MODY, particularly if the patient is young and has a family history of autosomal dominant inheritance.

**Correct Answer:** C.