What sex positions can help men maintain an erection?

Whatever Works! There is no specific position that will better enable a man to maintain his erection. Most people have particular desires regarding their "favorite" positions and it's a good idea to stick with that one if you're having erectile difficulty. A constrictor (aka "cock ring", which can be found online or in a sex shop) properly positioned, is a natural way to help with erections.
Depends - experiment. If the main problem is maintaining enough blood flow to the erection, passive positions (reclining with partner on top, side-by-side) are usually helpful. If his problem is being less responsive overall, whatever position(s) he finds most stimulating or exciting will work best - every man has different tastes. Men tend to be visual creatures, so an arousing view of his partner always helps.