What is his age,

Has this just happened, or has it been going on for a while ?
If for a while , has it gotten worst.

Is there good and bad days ?

What type of medication is he on, many things like blood pressure medication can cause issues.

Is he under a lot of stress, work, money , others ?

Also once a man has a few failures, they get worried and upset and it causes it to be worst.

Has he seen his doctor to rule out a prostate or other health issue

I'll second what Fr_Chuck said... the male hydraulics is a complex mechanism, everything has to be in order.

Regardless of age he needs to discuss this with his doctor. It could be symptoms of an undiagnosed medical issue.

First you need to consider any medical problems and/or medications that he may be taking. Simple medications can cause erectile dysfunction. If it's a consistent problem when he's with you or masturbating alone, he should see a physician.

If the problem only exists with you, but he has no problem getting and maintaining an erection when alone, then you need to explore the things that are getting him aroused when he's alone. These may be some type of exotic porn, ways that he touches himself, etc. Ask him what these are and don't feel embarrassed or insulted by any of them. The things that make us aroused change throughout our lives.