Familial Hypercholesterolemia: This condition is characterized by very high levels of LDL cholesterol, which can be inherited in an autosomal dominant pattern. The patient's LDL level of 3.7 mmol/L is elevated, suggesting a possible genetic predisposition. Early onset and high levels of LDL cholesterol are key indicators for this diagnosis.

Dyslipidemia due to Lifestyle Factors: High dietary intake of saturated fats, lack of exercise, and obesity can contribute to elevated total cholesterol and triglyceride levels. Although the patient is young, lifestyle factors could already be influencing his lipid profile.
Familial Combined Hyperlipidemia: This is another genetic disorder that affects lipid metabolism, leading to high levels of both cholesterol and triglycerides. The combination of elevated total cholesterol and triglycerides in this patient makes this a plausible diagnosis.

Hypothyroidism: Although less common, hypothyroidism can cause elevations in LDL cholesterol and triglycerides. It is crucial to rule out hypothyroidism due to its potential impact on metabolism and overall health.
Nephrotic Syndrome: This condition, characterized by excessive loss of protein in the urine, can lead to elevated lipid levels. It is a less likely cause but one that could have significant implications if missed.

Lipoprotein Lipase Deficiency: A rare genetic disorder leading to very high levels of triglycerides. While the patient's triglyceride level is elevated, it is not as high as typically seen in lipoprotein lipase deficiency.
Apolipoprotein C-II Deficiency: Another rare condition affecting triglyceride metabolism, leading to elevated triglyceride levels. This diagnosis would be considered if other more common causes are ruled out and the patient's triglyceride levels are found to be significantly elevated upon retesting.