I'm not a doctor, and I'm not an expert in sex. My thoughts might be simplistic.
But if they might help:
One thought: Technique:
When you feel like you're getting ready to cum, don't shoot as soon as you -can- shoot. Hold it back for as long as possible.
Another thought: Blood flow:
Maybe try one of the -denafils? Like Verdenafil or Sidenafil?