Thanks for the reply. My friend has been walking him sometimes as well, and he is just as bad, if not worse, with her.
I got a new kind of treat for him today that he's pretty nuts about (freeze dried liver), so hopefully that helps motivate him.
I'm thinking I might try using the off leash beach area as a reward for walking a couple of blocks around the neighbourhood first. If he refuses to walk any other way (with motivation from treats), I'll just bring him inside and try later. Then, if we have success walking up the block, then I'll loop around and take him to the beach for a little while.
The other half of the problem, however, is that he is starting to refuse to leave the off-leash, whereas before all this started he was absolutely great, and would sit as soon as he saw me starting to get the leash ready to put back on. I've been trying to get him leashed up at a random time, rather than when we reach the end of the path, and reward with treats if he doesn't duck into the bushes and hide, as has become his habit.