My boyfriend is petrified of marriage,why?

My boyfriend and i have been together 3 years with a 4month old son. Well i keep giving him hints about me wanting to marry him and i know he loves me he is madly in love with me and he shows it and proves it every dam day just wish he would marry me i don't want to be a life long Girlfriend. Well every time i bring it up he starts having like a panic attack and he is so petrified and i do not understand why he won't tell me and i have asked him and he knows he can open up to me just so unsure what to do, Advice?