Venous Leak: This condition is characterized by the inability to maintain an erection due to the leakage of blood from the penis, which aligns with the symptoms described, such as erections subsiding when standing and not lasting.

Peyronie's Disease: This condition involves the development of scar tissue inside the penis, leading to curved, painful erections, which could be related to hard flaccid syndrome.
Arterial Insufficiency: Reduced blood flow to the penis can cause difficulties in achieving or maintaining an erection, which might explain the symptoms of erections not lasting.
Neurogenic Erectile Dysfunction: Issues with the nerves controlling erection can lead to problems with maintaining erections, potentially related to the symptoms described.

Testicular Cancer: Although rare, testicular cancer can cause a variety of symptoms including changes in erectile function. It's crucial to rule out due to its potential severity and the patient's age.
Spinal Cord Injury or Disease: Any condition affecting the spinal cord can lead to erectile dysfunction among other symptoms. Early diagnosis is critical for treatment and management.
Endocrine Disorders (e.g., Hypogonadism): Conditions affecting hormone production, such as hypogonadism, can lead to erectile dysfunction and should be considered due to their impact on overall health.

Pudendal Artery Thrombosis: A rare condition where a blood clot forms in the artery supplying blood to the penis, which could potentially cause erectile dysfunction.
Fabry Disease: A genetic disorder that can affect many parts of the body, including causing erectile dysfunction due to the buildup of a particular type of fat in the body's cells.
Cavernosal Fibrosis: Scarring within the penis that can lead to erectile dysfunction, which might be considered if other causes are ruled out.