Walk towards him from the side, talking softly, so that he can see you approach and isn't startled. Pay attention to his reaction to you at all times. If he is wary or frightened, don't be in a hurry to get close - just behave calmly and quietly, using 'advance and retreat' if necessary. If he is irritable or aggressive, don't be scared and shrink away but try to remain confident and respectful of his feelings. Don't be intimidated by his large size - you probably seem much bigger to him than you realize. When close enough, allow him to sniff your hand - but don't pressure him. If he is sufficiently relaxed with your presence, you can stroke his face or neck. You can also give him a treat at this stage. Once he is happy and relaxed, go away - leave him alone to mull over the fact that the 'little, two-legged critter' really wasn't all that bad after all. Next time you approach, it will be easier. Your confidence and his confidence will grow each time you approach him.