Positive socialization will be the thing that will help him overcome his fear now. He may have fear aggression from his experience. Lily is still recovering from a year ago attack. She will react with aggression if another dog even slightly shows any assertiveness. Lily has the attitude of "Gonna prove I am tough before you can show me I'm really not".
Please work's posting summed it all up. Gonna take some time and patience, but you can help him overcome this and hopefully be back to his usual social self. Let him have positive experiences with a quick hello to another dog and then walk away before he has any reaction. He will learn not every dog wants to hurt him. But try your best to get him to say hello and then take him away before any reaction. Let him end on a good note each time. If a reaction happens, then slow things down and be prepared for him to go backwards if a negative reaction occurs.