Yes, neutering him would probably help, but it wouldn't miraculously solve the problem. Adolescent males his age literally have about ten times the testosterone as a stable adult male, so neutering him now could indeed reduce some hormonal reactivity. Just don't count on it to solve the entire problem; neutering him will not replace much-needed training.
Learn about calming signals so that you can recognize them in your dogs and use them yourself:
http://www.canis.no/rugaas/onearticle.php?artid=1
Learn about canine body language:
http://www.wagntrain.com/BodyLanguage.htm
http://www.wagntrain.com/BodyLanguage2.htm
Read this article:
http://www.clickertraining.com/node/2327
Suggested books:
"Click to Calm: Healing the Aggressive Dog" by Emma Parsons
"The Cautious Canine" by Patricia McConnell
"How to Right a Dog Gone Wrong: A Road Map for Rehabilitating Aggressive Dogs" by Pam Dennison
"When Pigs Fly!: Training Success with Impossible Dogs" by Jane Killion
You should strongly consider seeking the help of a trainer:
http://www.mastiffonlinecommunity.net/viewtopic.php?t=4276
Here's how to introduce two dogs safely:
http://www.mastiffonlinecommunity.net/viewtopic.php?t=86
Look into training classes for reactive dogs. They're often called "Feisty Fido" classes.
Condition him to associate the presence of your other dogs with good things.
http://pet-training.suite101.com/article.cfm/how_to_help_a_dog_overcome_a_phobia
http://dogs.thefuntimesguide.com/2010/04/frightened_dog.php
Replace "thunder" with "other dogs."
Good luck!