Your dog probably was not trained properly or did not get enough attention. He might think the other dogs are a threat to him and the people surrounding him so he goes by his instinct and attacks. It is never too late to train but if you are short on time at the moment, make sure you do not take him into situations where there are a lot of dogs. Ease him into it. This way he learns how to behave around 1 dog and then 2 and so on and so forth. I hope this helps you.