Recognize that serving in the military typically means adapting a new mindset of obedience and comradery. It’s a very different way of being, as the normal level of independent thought and action is better sacrificed for teamwork. This probably won’t have a direct impact on your relationship, but it’s a good example of how serving in the military changes a person.
You and your boyfriend can both adapt by treating his time served in the military as a journey into a new, foreign land with its own languages and practices.