My hubby is the same way. Skinny with a small appetite, and he loses weight when he's stressed. Wish I were that way!
I've found the snack to send him to work with is Doritos. He'll eat a couple of individual bags at work, at 150 calories a bag. He'll eat some at home, too. Healthy food is important, but when you're dealing with someone who doesn't eat much and you just want to make sure they get enough calories, resort to whatever you can to tempt them to eat.
If your hubby likes cheese or cookies or chips, I'd keep some of those around and send him with some to work. Anything that he likes so much it will tempt him to snack even if he's stressed or busy will do the trick. Maybe encourage him to go for a milkshake at lunch or make one for him after dinner. If you pack a sandwich for him for lunch, make sure to add cheese, and even top it with bacon if he likes that, or use mayo or whatever else he likes that's high calorie.
There are lots of things you can do to make foods fattier at home, but of course you end up eating it, too! Like use butter in dishes instead of a low-cal alternative, have mashed potatoes with butter and cheese, use full-fat milk and cottage cheese, offer up his favorite dessert and add whipped cream or ice cream on top. If you have salad, use the full-fat dressing and add croutons and cheese. Good luck fattening him up!