
Hi Pottydog
I think a bit more info may be required please:
1. How long has this behaviour been going on?
2. Is it just hackles up and noise or does he lunge at the other dogs as well?
3. Does he do this to ALL dogs regardless of their sex or just to other entire males?
4. What happens when he does get up close with other dogs and they are both off leash?
I don't know what breed you have but it could be that at 18 months he's not really sure of himself and is trying to push things with other male dogs. It could be that he's a bit fearful and trying to get in first.....has he been attacked in the past? If the behaviour only happens on leash it could be due to the fact that you are meeting another dog head on which is very confrontational for dogs. (They wouldn't choose to meet like this if loose, but by keeping them on a leash we are restricting/limiting their options as far as body language goes.) It could be that you are getting a bit tense and anticipating things and that tension is going down the leash. It could be...........well, I guess there are a multitude of possibilities, some of which I've alluded to above. Some more info may help, but this may be one of those situations where it is really difficult/impossible to suss what is going on without actually seeing the dog in action and that your best course of action may be to get hold of good behaviourist.