It might be worth starting a new thread, Hedieh, as this one is rather old. It sounds as if Lappo is demonstrating fear - "Go away!, Go away!" - in his barking. If you know any really kind, friendly, long suffering dogs whose owners will work with you, socialising him with them will help enormously - the key is to work at his pace, and never push him to the point where he is fearful enough to bark.
Meanwhile, I would try and keep him at a distance which does not trigger barking. The moment you see a dog - or he begins to tense and think of barking - turn and walk in the other direction. If there are a lot of dogs where you live, you may need to find a less populated area to walk him in for a while. Practice just hanging out at a distance from the other dogs in a park or cafe, claiming his attention and giving him treats - if he does not want them, he is too anxious, and you need to move further away. The aim is to combine gradually getting him used to remaining calm around other dogs, moving slowly closer as he gains confidence, with him associating other dogs being around with Good Stuff for Poodles, so that his reaction to seeing one is to look to you for a treat rather than to tense up and bark out of fear. Look up BAT (Behavioral Adaptive Therapy) for more detail on how to do this.
And whatever you do don't punish him for barking, however tempting that might be - it will just make him even more determined to keep them at a distance - "Go away! Go away! If you come any closer my Mum will be angry with me! Go AWAY!".