maybe he just don't like to hold hand with you. anything wrong with your hand? rough or not? sweaty palm? oily hand? or you ate something using your hand and didn't wash so you dirty his hand indirectly. are you short? cause maybe if you are too short and he is tall, he will feel uncomfortable holding your hand and therefore, refuse to hold your hand.

if you answer no for all the above, then your bf have Chirophobia - it means the fear of hand
http://healthmad.com/mental-health/inside-look-chirophobia/