P.M.
He's at an age where children can experience separation anxiety. Since he's apparently a pretty sensitive, attentive, and imaginative boy, your arguments may conceivably work into his complex feelings, perhaps as a subliminal worry about losing one of you.
I have several ideas that may help your son. First, don't deny his worry – empathize with it. Then he'll be able to process it and consider more positive possibilities. Start with "Oh, sweetie, you're worried about your daddy, aren't you? You are afraid something bad will happen to him while he's gone, right? Yes, that's hard. We all have worries like that sometimes. Is there any thing else you want to say about it?" Listen and affirm his statements without exaggerating it if he says any more.
Share with him that you miss daddy, too. And you're looking forward to his return. Show him a calendar, and mark the day his daddy will be coming home. Help him count off days.
He might also like seeing a map so he knows where his daddy is physically right now.
Hearing his dad's voice on the phone might help (it can possibly cause even more longing, though). If it seems reassuring, try to arrange opportunities. Your husband could even leave a recorded message about looking forward to getting home to his family again, and you could play the message occasionally.
Plan a welcome home party for daddy with your son's help. Give him something positive to do about the absence.
Make up a song about your son traveling and then coming home again. Make it silly. When he's good and involved, include verses about you traveling and returning, and his little brother, and any pets you might have, and finally his daddy. This may give him a sense of normalcy about travel for everybody, including his dad.
If he expresses ongoing worry about something more specific, such as cars having accidents, agree that sometimes cars do have accidents, and that's why people learn to use them carefully. Because people learn the rules for driving, they usually don't have accidents.
I'd also pay attention to whether either you or your husband tend to back-seat drive, express anxiety about the other's driving, or yell about driving mistakes. Or whether he plays crashing games or sees videos/TV with car crashes. All of these could have given your son a sense of heightened risk regarding cars.