My dog barks and growl at other dogs can anyone help?

you have only had him a few days and you said he come from a rescue, thats the only life he has known up to now barking and jumping up and basically trying to get attention over every other dog, he probably isnt being aggressive more than likely more scared than anything or hes just being a guard dog, what the best thing to do is just keep walking him everyday the more he sees hes going to get used to it and not bark as much at it anymore , hes just breaking in really. what you could try and do is take either a jar with something like rice in it and when he barks shake it to get his mind out of thinking like that, or take a spray bottle with water, and you have to keep it hidden so he doesnt know your doing it and everytime he barks, give him a spritz you have to make sure he doesnt see you if he does its all ruined. he has to think its a consequence for him barking.