First, I don't think the Reverend is saying he didn't expect his son to go to war. However he is expressing his view that he thought his son joined the Army for some other reason, GI bill, training, some extra spending cash from reserve duty.
This is going to be a longer than usual post so get comfy or tune out now

We all know that there are essentially three parts to the Army. There is the Regular Army (RA), these are the guys that are in uniform every day. The is the Army Reserve (USAR), most of these guys were sold on one weekend a month and two weeks a year by their recruiter. Finally there is the Army National Guard (ANG). Basically the same as the Army Reserve, but Guard units are sponsored by their home state.
Most combat arms soldiers are RA, so the infantry grunts, artillery, combat aviation, and armor for example are largely RA. There are some USAR and ANG units that are combart arms units as well. However, USAR and ANG soldiers typically pick up what are called Combat Support (CS) and Combat Service Support (CSS) functions. This would be your truck drivers, maintenance personel, signal units (communications), cooks and truckdrivers just to name a few more.
So it's likely our young truckdriver in the story was a USAR or ANG soldier expecting to do his 12 weekends and 2 week tour and cash checks, get GI bill.
I don't think you can say the Reverend had the same expectations. Maybe, but all were adults and had the chance to ask questions of someone other than the recruiter before signing up.
But even if the Rev. initially expected it to be that way, its pretty clear he wasn't blaming the war on what happened. He is blaming the chain of command.
This is a touchy issue, on one hand we want the Rev to fit in our box of what we expect out of him, stereotypically. On the other hand, you do need to examine what happened. Did a person in a leadership position make a fatal mistake? If so why?
If they made a mistake that got people killed, wouldn't it be best to take that person out of the leadership position? You don't want that leader to get more people killed, do you?
Of course, the person may learn from it, but do you want to take that chance with your son or daughter?
When I trained to be a young officer in college (I took an ROTC scholarship, my #1 objective was education, if war came along, I'd do my duty) we were trained to be decisive. Leaders had to make decisions and then we were critiqued on the quality of that decision. The training we got was to teach you to recognize situations so we could make the right decision. In some respects, there was a lot of emphasis on decision/leadership ability first, as I believe they had to find quick thinking people and then teach them how to quickly make the right decision.
Unfortunately, you never truly know if a leader will make the right decisions until you put him in charge. I'm pretty certain peacetime decision making is much easier than making the right decision in the fog of war.
So I don't see the mere questioning of what happened by the Rev here as being un-Christian. Perhaps he isn't going about it the right way. But then again, you and I didn't lose a son like he did.
So I think those who call for Christians to be forgiving, must also forgive the Reverand for such an expression of grief. Especially if we don't have the opportunity to speak with him personally and hear all of his words, and not just those that are quoted in some news article.
Thanks for reading along this far,
TB