Socialize more. There is no such thing as the end of socialization. Get people with other dogs to jog by and practice having him not lunge and not snap. If he does don't make a big deal out of it. Don't yell or get excited because then in his mind he thinkd well Mum or Dad is excited so i should be too. Stay very calm and redirect him (have him on a leash). Get a toy he likes and distract him. If they go by and he doen't bark praise like he just learned to flush the toilet. He IS the BEST dog you have ever met he is so good. The more he realizes that when he is quiet he gets attention the more he will play nice. If he keeps at it after the horses he could get a real nice kick and then you are at the vets. Make sure that for your first practice session he is tired. Take him for a good long walk first. Don't be discouraged if he is awful for the first 3 or so sessions. Don't give up, he will get it. He really has to understand what you want him to do.