Terriers!

Your dogs not acting like a Rottweiler at all, they don't need to act like that

He's acting just like most normal terriers, fiesty breeds, bred to hunt and kill, who are more likely to nip first and ask questions later if
not very well socialised whilst feeling threatened or having any anxiety, they are small so they get the message out they are not to be tangled with, the most likely breeds to be heard yapping in people's gardens too.
Your Cocker has different breed traits, unfortunately they will not rub off on the terrier.
What you are doing is correct, i.e stopping and praising him with a treat if he does not growl or bark, and then allowing a greet, continue with this, he will act worse on the lead as he will be more insecure, however off the lead if he starts on the wrong dog he may well start a nasty dog fight, so catch twenty two.
It is strange that he is beginning this well after maturity, he is well passed the initial increase of testosterone influences and fear insecurities in an adolescent, (terriers mature much earlier than other breeds) has he been attacked, has something happened? Has he recently been neutered?
He will need more socialising along with the praise and reward training you are already doing, (well done on that

) can you perhaps join a training class to help with this? Dogs also pick up on our body language, they can hear our increased heart beats and sense any changes in our biology, even a tightening of the lead will lead to anxiety from him, so try to stay calm and upbeat yourself.
Many people with terriers do have this problem, it is a real shame if he was not like this until recently, just work hard on making him feel secure again, and keep him focussed on you at all times, a good training class will help you immensely with these tactics too.
Wishing you luck, I often see people with terriers looking apologetic at their dogs actions, it's just terriers

can be difficult to combat, but socialise, socialise, socialise, and make sure he is not
ever in a position where his fears are made justified, that is just too hard to turn back around, perhaps that has already happened?
Good luck, and try not to stress about it too much.