as mentioned above, put him on a 20 ft long piece of light rope, and take a bit of his favourite tit bit,
ie cheese or a bit of cooked chicken with you in a polythene bag, every now and again call him back, whilst he is on the long lead, and whilst rustling the bag, when he comes to you pat him praise him up a bit, and give him a little bit of food,
only do this occasionally though, throughout your walk, or he might get bored with this little game. and start ignoring you.
do this each time you go for a walk, until he gets used to coming back, dont try to enforce to much training