He is on active military duty, so he is limited to what he can use. I'm sure ladyreem will convey the info. to him, or tell him to go to the infirmary at his post for treatment.
In the meantime, alternate wearing boots if he has more than one pair and wearing clean/dry socks (if possible). You can ask him if you are allowed to send over the counter medications to him, but I believe powders are not allowed to be shipped (but you can check on this). That is why I suggest he go to the Infirmary at his post, plus he will be treated quicker.
To help get the moisture out from his boots/shoes, you can put Bicarbonate of Soda into the footwear. This also eliminates any smells. It will not get rid of his itchy feet, for that he needs the antifungal cream.
Would seem he has athletes foot. He needs to keep his feet dry, clean, and before he puts on his shoes, have him sprinkle some athletes food powder in them. It is a fungus, so the drier he keeps his feet the better.
If you are from the US, you can buy over the counter cream and powder (Lamacil) over the counter that you can send over to him or he could buy where he is deployed.
If he has more than one pair of boots, he should alternate wearing one pair of boots on one day and the other on the other day thereby allowing the boots to dry out in between.
It is likely that your fiancé has athlete's foot. This is caused by a fungus.
You can get a cream for athlete's foot from the pharmacy and also a medicated talcum powder for it too.
He really needs to wash and dry his feet thoroughly and wear clean socks every day, which I appreciate is not always possible for him.