Barking dog problem

[Jules001]

Our dog is 3 years and 8 months old,( Bichon/Lhasa cross) we have only had him for 6 months rehomed from Blue Cross full history as previous owners were unable to take care of him any longer and was only at Blue Cross for 2 weeks before we got him. He has been well socialised with other dogs and walking him has always been a pleasure until fairly recently, he seems to have a problem with dogs that are about 5 - 6 months old and chases after them whilst barking at them. To me it doesn't sound like a 'I want to play' bark but more of a 'telling off' bark. He completely ignores me calling him back when he is doing this and has normally got excellent recall. I put him back on lead when he does this (as soon as I can get him that is), it has now got to the stage where I will walk at different times to avoid the dogs he has a problem with, but this isn't the answer and wonder if anyone can maybe shed some light on why he is doing this and what else I can do to stop this behaviour.

[tamborine]

he's warding off other dogs from you as you are his territory. Keep him on an extendible leash when out till he calms down.