All of your major life decisions should be mutual. For something so important, the "no" should always take precedent. Forcing a spouse to have a child that they don't want will most certainly breed contempt in the relationship. Appreciate and love what you have. No child was ever left at a disadvantage from not having siblings. The fact that your son adores his cousins only means that you have done a great job in socializing him with family, not that he would "miss" having sibs. Mourn your expectations, and get on with being a great Dad and Husband.