Try something new and different from anything you've ever done before. If interest is not the problem, and things get a little "soft" there might be something physical causing that to happen. Hormone levels, blood pressure issues, and some medications will cause lack of interest or firmness issues.
I think your best bet is just to go visit a doctor. They can prescribe medicine, and know the right kind of medicine for him to take instead of trying random things that might not work, or cause a bad reaction to him. Also talk to him if you haven't already and see what he thinks about it all, maybe something else might be bugging him making it hard for him to keep an erection. I know when my husband is under a lot of stress it is harder for him.