I'm 19 years old, and my partner is 28 years old. We're both very active and physically fit. There is a definite sexual attraction. He is always wanting it, and majority of the time, there is no problem. But sometime he goes limp. Sometimes we can get it going again, but then other times, we cannot continue. Is there anything I can do to help this? Different position? Or motions? Anything.

If it's infrequent, I don't think you have anything to worry about. Even when fit, healthy, and young that sometimes just happens. If this is a regular problem, then have him see a doctor.

Does he drink or do any recreational drugs? They can have an affect on the ability to perform.

Pay attention to what is going on when it happens. You might find that the room is too cold/hot, ate too much, didn't eat, too close to the last time he ejaculated and needs more 'recovery' time, too stressed out, too tired, etc. Even a minor injury can be a libido limiter. If you find that there is a reoccurring distraction, then you will know what to watch out for at other times.

Do not make a big deal of it when it happens. Change the focus from sex to non-sexual intimacy. Enjoy being together.

If it is happening more often and he seems bothered by it, he might want to get a check-up. Some diseases such as Diabetes and high blood pressure can cause erectile issues even before they become major problems.