The laws may be different here in the UK, but if it were me I would box up his stuff in a strong box and write his name on it, so there is no intention of keeping it, and wait until the opportunity arises to give it back - perhaps when he is home on leave. I'd have thought he needs his dog tags with him, serving soldiers are supposed to wear them all the time especially in war scenarios as he appears to be. We have specialist postage rates to soldiers overseas - isn't that the same in the US? - so it might not cost as much as you think. You're not stealing as long as you keep his stuff separately and have every intention of giving his stuff back.