Need help...

My husband and I have been married for 8 years now. There is a 15 year age gap between the two of us. I am 34. He is 49. We have a blended family with the youngest being 15 years old. Before we got married, my husband and I agreed that we would have a child together. I am now so ready to have a child, and now my husband does not. He feels that he has gotten older and will not have the energy to raise a child. We have discussed this endlessly with neither side wanting to budge. At this point I don't know what to do. I love my husband but I can't get rid of this desire to have a child. Any thoughts on how to deal with this?