I have a one and a half year old bichon cross. He is the most lovely dog, and was such a sociable puppy. Up until the past couple months, he was so focused on other dogs, always being over friendly with them, and generally obsessed with any he meets on his walks. We kept him leashed on all walks until we got the 'come' command mastered, and he managed this very well- however would always be running up to any dog and trying to play. For the past couple months now, he seems so fearful of dogs he doesn't know. In the park if he is not on a lead, if another dog tries to come close to him (by running up, which he did himself not too long ago) he will take off in the other direction and will not come back. This is happening more and more frequently now, so we keep him leashed more often.
Today at the park, a beautiful pug was eyeing him up, and my dog was staring at him from a distance (off lead) trying to suss him out. Eventually, they both went up close to each other and they had a great time running about and playing. About 10 mins later another pug appears and starts to join in the play, but my dog is all of a sudden so fearful of them both (despite playing so well with the dog previously). Before he had been fearful of bigger, energetic dogs, but these dogs were half the size of him and were playing together so well.
I'm at a loss what to do. He seems to scared, but for no sensible reason to be scared. The dogs he runs away from aren't acting aggressively, and more so just looking to play, which he was so eager to do not too long ago. There isn't a turning point in when this behaviour began. He was not attacked or bitten by another dog - he always seemed so oblivious to when dogs growled or snapped at him for him running up previously. He is not like this with all dogs, but he does take his time to suss them out and to figure out they aren't a danger.
I would love to get him back to the confident wee doggy he was before :( Any help?