How can I get my dog to stop being aggressive towards dogs and men/boys?

First things first, is he neutered? If not, save up the money to get him fixed, you would be surprised at how much that makes a difference in your dogs behavior! Second, the Lhasa Apso breed is known to be nervous and even aggressive around other dogs, strangers, and small children, of course this makes them untrustworthy so you will always need to keep an eye on him in situations that he might bite someone or another pooch. Often times socialization as puppies will help tone down this behavior but since you said that he is an adult now your going to have your work cut out for you. Here are some things you can try. One is positive reinforcement. Boil some chicken and invite a male friend over. (Make sure this friend of yours is patient and willing to help you.) When your friend arrives make sure you have a leash on your dog. Go sit in the living room or another room where there is a doorway that your friend can walk out of so that your dog cannot see him. Once you have found this spot make your friend wait out of sight. Chances are that your dog is going to know he is there and bark even though he cant see the man, that's okay. Have your friend come into view, as soon as you know your dog sees him make a distracting noise such as a high-pitched squeak to get his attention. This will make him stop barking for a split second, give him the treat immediately and have your friend walk back out of sight. You dog will probably go right back to barking so repeat the process over and over again. Eventually he will see your friend and instantly look to you for a treat! Once this happens move onto the next step. Have your friend step in the room and give your dog the treat just like earlier only this time have your buddy stay insight after the treat has been rewarded. Wait a few seconds after you give him the treat and have your friend take a small step towards you. If your dog does not bark automatically reward him. (Something I forgot to mention, speech is important to praise, praise him in a height pitched squeaky voice, they understand voice pitch more than you might know). Keep rewarding him for every step your friend takes without a peep from him. If he barks at any point send your guy back out into the hall and start again with the entrance and the one step at a time phase. Eventually your friend should be able to sit within hands reach of the dog without a reaction. (Be sure to keep his leash in your hand to prevent any unexpected attacks!) Once your friend is sitting within touching distance of the dog hand him the treats. Have your friend hold a treat in both hands but only offer one treat to the dog. The treat he offers the dog should be in a lose closed fist with the back of his hand pointing towards the ceiling. When the dog leans forward to get the treat have him slowly offer him the treat in the other hand (not the hand that he has held out to the dog.) When the dog accepts the treat without a reaction praise him. Continue doing this until the dog is comfortable enough to walk up to your friend to receive a treat on his own. After doing this for a while have your friend leave and come back in a few hours, or you could even get another friend to help you. If needed start over from the beginning. With persistence he should start realizing that strangers are a good thing, after all every time he sees someone new and doesn't react he gets boiled chicken!! When it comes to other dogs, you can do the same thing, have a friend bring a very docile dog to your house, one that would run rather than fight. Every time he sees the dog and doesn't react reward him! Keep starting over until they get close enough to smell each other and then start over. Remember he needs to stay on a leash. Take him for walks in a dog park, and go on one of the lesser used trails. Every time a dog passes and he doesn't react reward him, if he does react make a distracting noise so you can get his attention, as soon as he starts looking at you instead of barking at the other dog reward him. You should soon be able to cross another dogs path without a reaction! Keep in mind, these dogs are known to be aggressive so you wont ever be able to leave him unsupervised in an environment where he might attack. Also once you start this training keep up with it, always keep fresh boiled chicken or some other tasty but healthy treat around. I wish you luck! Let me know if you try this and if it works please!