My husband hasn't made love to me in four years

I have been married for 5 years and I have a four year old son. The problem is my husband has not kissed me or made love to me in over 4 years. I have tried everything but he just wont do it. I cry myself to sleep most nights after he rejects me. Apart from this, everything else is fine and our friends and family have no idea what's going on.
I need advice. A. what can the reason be ? because I honestly don't think he is cheating on me. B. what can I do about the situation cos I'm feeling like my youth is wasting away. I will soon be 30. I married him at 25 and I'm like is it when I'm 45 he will be ready to get freaky. All my friends now have 2 or 3 kids but I still have 1.
I'm just so confused i haven't cheated on him (I have thought of it) but I just cant. I have prayed, I have fasted, I have cried, I have begged, I have asked him so many times forget the sex, just give me a reason, just tell me why you wont or tell me why you cant. He still has erections when I touch him so he's not impotent so I really cant explain it at all. I'm writing you now after another rejection, tears rolling down my face as I type. Its just heart breaking. I hope you and your readers can help me before I go crazy or do something horrible.