Thank him for his service.
I will, and that's appreciated.
My husband served in combat as well in Iraq '03-'05. So our progressive family is not immune from the hell that is war. Having to get a message from my son that he believed was a goodbye letter to me as his mother he loves deeply is also an experience not many mothers share in. But opinions aside, I'm sure other moms can imagine and sympathize and still stand firm on their ethics.
That being said, if a trans person is fit to serve, then cis people who are soldiers and sailors need to get over whatever assumptions and prejudices they may carry into battle. If protecting a soldiers prejudices is more pressing than protecting the unit, then not only would that jeoparize the mission, but it would put troops at greater risk.
My son or my husband should not be regarded as reasons to pause over because they are socially progressive and atheists.
I'd expect the same for people who don't fit the stereotype of a ready military - white, male, straight, Christian, cis.