I have a chocolate male lab aged 14 months.  We got him when he was 8 moths old to join our 2 other male labs.  When we got him at first he would try and mount the other dogs but after a week or so he stopped.  He has now started to mount our yellow male lab and out pops his manhood.  I am not sure why he has started doing this.  I also have 3 daughters and he won't stop trying to sniff their bottoms, but our other 2 labs have never done this.  Can anyone advise what is best to do.  Thanks

It is fairly common behaviour for a 14 month old dog, the fact your others have not done it may be because you had them from pups and taught them better manners.

Would just make it known to him that you don't want him to do this, is the yellow lab castrated by any chance, if he is it could be that he smells like a bitch in season and that will cause an intact male to behave in this fashion.

Ditto Nova's reply.

Distract him from behaviours that are unacceptable - treat him with the same patience and determination you would were he a young pup and you'll soon find that he stops doing these things :) 

regards, Teri