I don't want to sound 'ageist' - but since I'm substantially older than you, I'll give my perspective.
As a young man erections happen far too frequently - it can be embarrassing. But with the ravages of time it gets to be more of a challenge. Once you reach the point when you have difficulty - then your head gets into the act and anxiety makes it even harder.
Older men trying to keep with their wives - especially if they are younger like mine - is a challenge. Believe me sex for an older guy is far ahrder than an older woman. The most common problem for a woman is dryness - throw on some lube and you are ready to go!
Back to your SO's difficulties - here's what I do:
I wear a ring most of the time (my favorite is the Omega). It doesn't so much help you get an erection - but it does help you keep one. It has the side benefit of making you a little larger because of the retained blood.
I'm not afraid to take cialis or viagra as is necessary. Cialis works best if you're not sure exactly when you're going to have sex. If you know when and take viagra 30-45 min in advance - it works fantastic.
A little social lubricant i.e. alcohol to help leave the outside word and anxiety behind. This is a double edged sword - one too many and it'll have the opposite effect. For me 2 beers is perfect.
Have him checked for testosterone levels. Mine were low and so I've been prescribed a bioidentical cream - it has worked wonders for me.
Be patient - sex with an older man required more time and firmer more direct pressure than someone younger. Pay attention to his breathing and try to help build his self-esteem.
Lastly - if he's open to it, a finger or two in the ass during a BJ sends me - and many other men into another place - a good place for orgasms. Educate yourself on proper lubrication and technique.
Good luck - and don't lose hope!