Normally the prescribed amount of Sildenafil to treat PAH is 20mg three times a day. My understanding is that when treating erectile dysfunction the dosage is normally 50 to 100mg taken all at once. So I would guess that your BF not having any reaction to taking Sildenafil may have everything to do with the dosage and the frequency. He would likely have to take a higher dosage to have any effect. If this is a concern, perhaps a conversation with his PAH specialist might be in order.

And there can be many other reasons causing ED including testosterone level, etc. Best to talk to a doctor.

I believe there may be other health issues at play here! Talk to your physician!

Sildenafil was originally prescribed for cardiac conditions. It was discovered that a side effect of the medication was an erection for some men. I would venture to say that it might cause your boyfriend to have an erection. Why don't you talk with his doctor about this subject at his next appointment?