A very difficult one for you, meglet.
And there isn't really a compromise.
They are funny sorts, these lads who seemingly can't wait to put themselves in harms way............and you ARE right, there is something that seems to be hard-wired into them.......
I think for your relationship to survive, you have to let him go. He WILL resent it (and you) if you stop him and there is every chance that your relationship will wither away, in any case.
If you can't bear the thought of him going, then perhaps you need to disentangle yourself from him now. For self-preservation, only.
Otherwise, tell him ONCE that you don't want him to go...........and leave it. Smile and listen to his plans; promise to write and keep to it and make the prospect of him coming home something that he so looks forwards to that he won't (hopefully) want to go away, again.
I presume that you weren't together when he last went away ?