13 mth Bernese MountainDog Fighting other dogs

Question
Thank you.  Regarding fixing him, I was following a strategy of keeping him intacted until his skeletals were fully developed, theoretically mitigating the arthritis/dysplasia probabilities.

We will try the muzzle...will he be vulnerable to the other dogs at that point?
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-----Question-----
I have an unfixed,13mth old male BMD.  He is on the feisty side of other BMD we've had and met.  Recently, when I've gone to an offleash, non fenced dog park, he plays with many of the dogs.  Unfortunately, he also invariably gets in a fight with another similiar sized dog (different dogs at different times).  He is a sweetheart to humans (friends and strangers).  Any ideas?
-----Answer-----
Well,first of all getting him fixed will reduce dog aggression ALLOT,and there's lots of other reasons,such as lower the chances of getting certain cancers,as well as preventing unwanted litters winding up in animal shelters as there's 4 million dogs killed year because there isn't enough homes(many are purebreds as well)

Also what works well for allot of people is,have him play with dogs he knows let them play,but put a muzzle on him so he can't hurt the other dog(and if it makes you feel better put a muzzle on the other dog too)but he will get used to playing with dogs and realizing they won't hurt him

However,some dogs will also be dog aggressive and allot of times it is incurable,but do try the muzzle thing at least

Answer
Yes for large dogs they do often tell you to wait to get them fixed until about 18 months-so you are right about that

Now if you are afraid of him being hurt while muzzled-what you can do go in a yard of a friend's dog,and ask them to muzzle there dog as well(or wherever you are)that way they both can't bite each other-that is why it is best to do this with a dog that you and your dog knows pretty well is unlikely to hurt your dog-but again if you feel safer muzzling both dogs go for it