His penis can't shrink inch by inch, but certain lifestyle choice and even age potentially reduce the ability of the penis to remain large. Lifestyle habit, such as smoking, and age can reduce testosterone production.
Smoking damages blood vessels, so that it disrupts the flow to the penis and thus, prevents it to get harder. Decreased testosterone can affect the strength of erection. Certain health condition, such as thyroid problem and diabetes, can also reduce erectile function.
If you measure the length of the erectile penis over time, it can seem to look as if it's shrunk in size. But any change in penis size can be difficult to measure. If you still feel why his penis isn't as strong as it used to be, he should see a doctor.
Although many causes erectile dysfunction, ranging from stress to performance anxiety, it can also be a sign of underlying condition, such as heart disease. Flabby penis also shows he needs to make lifestyle changes, like going to the gym and not stopping exercises.
Exercise can prevent narrowed blood vessels and also help increase testosterone level. Drug, like Viagra that works by increasing blood flow to the penis, can be an option if he can't finally erect again.
If his erection isn't as strong as it used to be, try not to stress and relax because it can make things worse.