HI there
You sound a bit desperate

:)
Good idea to get him checkedout by a vet, that is a great first step. If he is given the all clear, then what may be happening (and bear in mind that there may be many X factors here - because email communication isn't always easy) is that for some reason he associates the lead with something not very nice and is peeing for that reason.
If that is the case, then it would be up to you to work out what has happened, or what is happening, to make him do this. I am not an expert on Dachshunds at all, but I know they can get bad backs - could it be that he associates the lead with a walk, and it hurts his back? Or has he been accidentally jerked on it and it scares him now?
Just a few ideas to start anyway :)
Lindsay