I believe you should give him a positive answer like "yes, I will marry you", if at all you really love and want to spend the rest of your life time with him. Alternatively, you can just tell him to give you some time to think about it since marriage is a life time commitment.
If u want to say yes! If ur hesitant then tell him u want to wait.. It's definitely not something u want to be unsure about... Don't just do it out of guilt or pressure... Be smart and make ur decision completely on ur own