I dont think anyone else mentioned this, and perhaps you already do it? It may help to not always clip his lead back on, or finish the walk when you call him. Keep calling him back to you throughout the walk, when he gets to you, reach down and hold his collar, ask for a sit...reward, and let him go again. He should soon realize that your calling of him does not necessarily mean the end of the fun.
Another thing you could try is to take his lead and a long line with you on the walk. When you unclip the lead, attach the long line. He will think he is free and run, but as you have the line attached to him you still have control over him, and can use the line to encourage him back to you. As he gets used to this he will know if you have attached the long line or not, so you need to clip and unclip the line a few times to make him unsure as to whether he is still 'attached' to you.
Good Luck
sportsmonki