hi and welcome to the forum
when you come back from your walks why dont you take him straight into the garden and keep saying wee's then when his done one, make the biggest fuss of him saying goodboy and even give him a treat.
do the same when yur out as well tell him go wee
he will get the idea might take time but your get there.
i bet his a beuti
look forward to seeing some pic's of him.