My 5 month old Mastiff gets along well with my poodle but when on a leash at the pet store seems to become aggressive toward other dogs, raise his hair on neck. We have friends that bring their dogs over and he plays fine, is this because of the lease, I have heard dogs can be more aggressive on a leash due to the extention of their owner.
You dog may be nervous of strange dogs and many dogs feel the best defence is a good offence . in other ways if I act this way they will stay away . correct this behaviour , make him sit at heal when strange dogs are around and correct him if he barks etc. you need obedience classes for him he will get used to behaving around strange dogs at classes.
Its sounds like you dog is showing some signs of dominance. if your dog pulls you while on the leash, if he's the first one out the door, doesn't stop barking when told or drop any objec
(cont)object when told he's taken charge and will make any decisions he wants to until you take control and he sees you as a leader or in a dominant position. All dogs will see us in one of two positions, a leader or a follower. I own a bullmastiff and rottweiler. I control what actions they take and when they take them. consider reading two books, "cesars way" and "being a pack leader." these books will assist you in understanding your dog and it explains how leadership is so important to a dog.
my 5 month old boy started doing this as well i started taking him to the park and walking him i make him pay attention to me and by the second lap he was ready to take a break on a parkbench and watched two people walk by without a worry and then i stopped and visited with someone and he sat and stayed while i talked..... i think you should just get him out there while he's young and make him see new things are not scary and make him listen to you!!