It depends on why he doesn't want to go to the doctor.

If he doesn't want to go because of embarrassment you have to convince that it's not. That doctors have seen everything and are not their to judge.

If he doesn't want to go, because he's afraid of finding something wrong, you have to convince that it's better to know than to not. It could be something that needs to be taken care of and can be taken care of.

Keep trying.

Is this a recent problem?

Well It's been going on for about five months now. He just doesn't want to know what is going on and I'm very worried that it might be something serious.

He should definitely get checked out. Keep trying to get him to go to the doctors. You say he doesn't want to know, so convince that it's important to know and waiting could make it worse or too late. Find out if he has any other objections as well. Such as, does he not like his doctor. Female doctor? Will be embarrassed going to the doctor anytime after this? And find a solution, such as get a new doctor, go to a specialist, get a doctor of a gender he'd prefer, etc.

There are problems that could be diagnosed, but I'd almost think they're just going to diagnose ED and give him pills. The problem with that is, what is ED? It's not a disease, it's a symptom. So find a doctor that will actually conduct tests. Blood pressure and diabetes can cause ED.

The most important thing is to be healthy. Eating right and exercise can have major affect on all areas of well being, including erections. A naturopath could help you with that.