I haven't had to deal with younger siblings, but I do have a son in the Army. I worry about him (he's been on three tours of Iraq), but a military life was his choice, and I just have to deal with it. Happily, he did his "basic" back in college, and nowadays his wife keeps us in the loop about things that are going on. I even got to see (via computer) his unit's homecoming ceremony about a year ago when he came back from his last tour.
Ask your stepson to spend some time with his stepbrothers, telling them about what sort of things he'll learn to do and why he's going to like it. If they ask him about going into danger, he needs to be upfront about that. He can tell them that part of being in the military is being willing to lay your life on the line for your country, and that's all there is to it. But the soldiers all try to help one another to stay safe. They work together.
If HE can prepare the little boys for no communication from him for several weeks, that will be better than your explaining it. Ask him to give them a calendar (even if you buy it) so they can mark off the days until his training is over. Please ask him to write (yes, snail mail!), call, e-mail, or whatever his options are whenever he is able to do it, and ask his girlfriend if she will pass on any news she hears. If and when he has a mailing address, his brothers can send him pictures or drawings or whatever they'd like him to have.
Relatives of military people are in a better position, communications-wise, than they used to be. Far from not being able to hear from a husband/father/brother for months (unless the training or assignment requires it), it's possible to be in much closer contact now. My DS and DIL have Skype so that when he's away, they can actually see each other, and their baby daughter can see her daddy, too.
Meanwhile, let your little boys know how proud YOU are of your stepson for wanting to serve his country. Your attitude will be a huge factor in theirs.