I am currently looking after a 10 year old neutered border collie ( I am a dog minder). He arrived on Thursday & has behaved wonderfully until I took him out for a walk Saturday afternoon when he felt the need to keep humping my leg to the extent that I had to abort the walk. He is fine around the house but the minute we get outside the front door, he starts again. Yesterday I took my husband with me & he held the dog but Blaze still kept eying me up & if I got too close he would try again. In 8 years of dog minding, I've had the odd humper, but nothing like this. Anyone got any ideas as to why he should have started this & what I can do about it as it makes walks very difficult & I fear lack of exercise is only going to make him worse.

As it's only happening when you take him out  it could to be due to excitement of the walk, or, I wonder if perhaps he feels a little insecure while out and the humping makes him feel better, like a coping mechanism?

If it's a nervous thing then giving him something to do that he enjoys should help (fetch/tuggy toy/obedience work if he enjoys that). If it's an excitement thing then re-directing that excitement onto something else will help (fetch, tuggy toy etc.), so either way - disraction

Thanks for your reply. I think you may be right about him being nervous or insecure. I watched him this morning while I was getting ready to take him out & he did  a lot of yawning which I know is a calming sign. There must be something I am doing that upsets him. Possibly because he has to have his walks on lead. This morning I was able to use an extending lead as I didn't have any other dogs with me & he was much better. Hopefully I can sort this out as walking him on my own with 3 or 4 other dogs could be a nightmare.