Behaving different on lead

Hello everyone I havent been on for a while, been quite busy and havent had chance. Its been nice to read and catch up on some posts
Im just wondering if any one can give me some help with how Harley bahaves on lead.
Hes just turned 18 months old, and still intact, hes perfect off lead with all dogs never shown agression and when dogs have been the agressor hes never reacted. When he is walking on lead he lies down when a dog approaches and creeps forward and stares at them. When the dog approaches he gets up quick or lunges at them and some times doesnt look to sure.
A few times hes made loud noises like growling at the other dog and today he had his two front paws on a labs back and his hackles went up and he got annoyed and the lab snapped back. I think its mostly noise but it worries me as I do not want him becoming agressive.
I am also wondering if I should start taking him back to dogs trust training for the social aspect. What do you guys think? I will also put some pics up soon hes changed so much in the house much calmer and a real pleasure.He is also grown some more and started to fill out hes now a 19kg ball of muscle and my arms dont half know it! Just got to sort this issue out and was wondering if i should be firmer and not let him lie down but he gets nippy and jumps up if I pull him.
Thanks guys

Have a look at this:

https://staffy-bull-terrier.niceboard.com/t64420-bat-training-for-dog-on-dog-aggression-and-other-things

Chaos is pretty much the same, fine off lead albeit rather 'over enthusiastic' for many dogs which is why he's on lead when there are others around. He doesn't do the lying down bit, but the lunging and looking very unsure exactly him.

Since starting the BAT training, he's learning to relax much more. It sounds like what Harley needs too. Have a look and see what you think.

Thanks Liz I will have a look at that on a good note I took him to an indoor dog park today near where I live, it was brilliant Harley had a great run around with a dobe and had ago at the agility which he loves. The staff were very helpful made us feel very welcome. There was also indoor bits for nervous dogs and dogs that diddnt like others. Im glad I have found it and will be taking Harley once a week to socialise.