Sheltie (male) agressive towards other dogs

[wayne]

Hi
I have a75lb collie (female 6yrs) & a 22lb sheltie ( 3yrs). I have trained both dogs in obedience myself. They are for me fairly obedient on & off leash.
My problem I cannot seem to correct is when walking with the dogs and the sheltie sees another dog he goes ballistic. I have used a prong collar but any forceful correction seems to aggravate the situation. I have tried putting him in a down position & comforted him & this seems a bit better but he still growls & snorts.
He has several male & female canine companions that he has no problem with even in his home. It seems mostly certain dogs and when we go for a walk.
Any suggestions on easing this aggression he has towards other dogs would be great.
Thanks