Need Help

Hi I am getting next August and I am already having second thoughts about the man I`ve been with for the past three years. I am scared that when we get married that he will turn away from and start looking for somebody else. I know he loves me but since he asked To Marry Him and I said yes he has been acting different. Every time I ask something to do with the wedding he says he will talk to me about it later but later never seems to come with him.
Is this a typical guy thing or is he having second thoughts about our wedding.