First, you need to ask yourself what (if any) plans you and your husband had in terms of your family size. Did you agree to only one child or had you always planned to have a larger family? If you had once been open to having multiple children, your husband may feel blindsided by your recent reluctance. If this is the case, it may take some work to find a solution that makes you both happy. It might help ease your husband's disappointment if you show that you're keeping an open mind. For example, you may want to consider tabling the discussion for a couple of years. Toddlers can be demanding, and you may feel differently about expanding your family when your kid is a little older.
You should also think about why you don't want to have any more children. Is it because you already feel overburdened with one or feel like you don't have the energy to start over with a newborn? In most families -- even when both parents work -- the bulk of the childcare still usually falls to Mom. If this is true for you, maybe you can work out a plan to split things more evenly. If your concerns are financial (you were thinking of going back to work once your son started preschool, say, or you're worried about how you'll afford college for two kids), maybe you and your husband can come up with a solution after talking things through together. Regardless of the reason, though, it's important to let your husband know why. He may not have been taking your reluctance seriously or realize how much thought you've put into your position.