Well if you are young, perhaps in your mid teens or so, this is common and at this time not something to truly fret about.
With love comes responsibility and love grows over time, or for the worse you never really loved at all.
If you are young you are most likely not going to take the advice of an older person and just because I say what I believe to be true does not mean it would be true in your situation.
In my experience you learn as the relationship goes along and if it continues you begin to realize a great number of things. After you have helped your partner through the hardships and enjoyed the good times in your relationship you start to weigh the pros and the cons. There will be days when you are happy with the relationship and days when you may wish you had never met the person. You have to be realistic, love is not a smooth ride, there will be many ups and downs.
I could go on but I won't.
Give the relationship you are in much more time, try to be patient and understanding and see if his fear begins to subside.
Love takes time, sometimes a very long time to be truly realized.