Aggressive Collie

We have a 3yr old male collie (rescued a month ago). He doesn't like other dogs. We have two others too, a male and a female. I was hoping he would learn off them. We have had a few run ins with the two males and I thought they were trying out the pecking order as this happened with another male collie we had who recently died.

We have now just got to a stage where they get on but when out for a walk he particularly goes for black males and usually black labradors. I'm always worried about who is round the next corner. Even if I put him back on his lead (he has a great recall) I'm worried that another dog will run towards him (and they always seem to 'home' in on him) and then attack him. He is perfect in every other way, great with the children and very chilled in the house, and I feel I am pack leader - they respect me and listen to me and walk to heel. I want to nip this before it escalates.

When he does go for another dog I pin him down by the scruff and he understands he's done wrong because he goes all submissive but then he will do it again! I have also tried walking him on the lead around the village with my friend and her black lab male on the lead to try to de-sensitise him and make him bored by the other dog but that hasn't worked either - I don't like his body language.