I don't see an easy, obvious solution. This is much like submissive wetting. Start talking to him before you pick up the leash. Use a happy tone of voice. Act like you are really glad to be taking him out. Keep up the happy talk when you get outside. Continue to praise him for going outside. Keep him moving between stops to sniff. The exercise stimulates the need to go.
Try to build self confidence. Success and praise in obedience training is one way. The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog. Life is much easier if the 2 legged pack members
outrank the 4 legged ones. You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at
http://www.dogsbestfriend.com/. if he is already obedience trained, continue to work him, or move on the advanced obedience or agility.
Play tug of war with him and lose. At the end of the game, you take the toy or rope and put it up. Otherwise, you can undo the top dog idea.
Ropes from the pets' store quickly turn to hazardous shreds. Ones I made
lasted much better. Go to a hardware or home center that sells rope by the
foot. Buy 2' of 3/4" poly rope. Melt the ends, and tie knots in it. Get
them as tight as possible, put it in a vise and pound it with a hammer. Watch
carefully, and be ready to discard when it comes apart.