How do I keep him fat while getting me thin?

Add cheese to his double portions.

I'm serious. I had this same issue with my underweight, but physically employed BF. He ate twice my portions, but mine had no cheese, while his did.

i second putting cheese on stuff
give him creamy based dressings for salads when you use a light vinaigrette
if you eat egg whites at breakfast, give him some with yolks and toast with butter
give him full fat yogurt over fat free
are there any sweet treats he likes that you don't? for example my husband loves mint choc chip ice cream but i would never touch it so i wouldn't be tempted

Working construction burns an enormous amount of calories which your desk job doesn't. In order for him to maintain his weight, he would need to have far more calories and protein than you. The best meal for him to do this is breakfast. This will give him the calories he needs during the day. He should probably eat breakfast and lunch "his way" and have dinner "your way" as a nice balance that would work for both of you. Your change in eating habits is clearly working for you. Congratulations on your loss!

We have the same problem in my house, my husband is a long distance runner and has an active job, whilst my son is underweight due to a medical condition so we all eat differently.

Cheese is a good suggestion. When softening veg for casserole/curries etc, I only use a little oil and take my portion out, then add a bit more oil for them. They snack on different things and my husband eats twice the amount I do. I sometimes make him egg fried rice for him when I have plain rice. Salads with nuts and fattier salad dressings than you'd have on the side of meals?

Tea tonight, for son: fish, veg and new potatoes baked in the oven with pesto. Husband and I are having the same but with chicken and harrissa. He'll then have brown rice on top (he's running tomorrow). Pudding is rhubarb - my son with clotted cream, greek yogurt for me and husband. Later in the evening he'll probably eat the rest of the brown rice but he'd usually have some variation of crisps, olives, cheese and fruit in the evening.

It's a pain in the butt cooking separately, but you could make some batches of meals and freeze them, less of a pain on a weekday evening. Getting a little frying pan and a little casserole dish with room for one meal is really helpful.

I'm in a similar situation, cooking for an underweight and an obese (me!)

I make a lot of ommelets for brunch, and I give her all the yolks, puts lots of cheese on it, and serve it with bread an butter - I get the whites (plus a little bit of yolk for flavour - I usually do one yolk for every 4th white), no cheese, served on spinach leaves.

If we make chili's or tomato sauces we usually make them my way (aka lots of lean veggies, no oil, no cheese) but then she'll have pasta or potato and cheese, and whereas I'll put the sauce/chili (I'm not sure of the english word for the things you might cook with ground meat and tomato etc. and put on top of a bit of pasta, but usually it's the stuff on top that's the main part of the meal?) on veggies (broccoli, cauliflower, squash, eggplant)

Sometimes I'll bake spinach wraps for me, and she'll have store bought tortilla wraps, and I'll give her more cheese and more meat (unless it's super-lean - then I'll give us equal), and dressing, and I'll get more veggies, no cheese, no dressing (though sometimes a bit of low fat greek youghurt)

Sounds similar to me and my husband. He has good genetics and naturally lean. Recently he went for hypnothery and a really random side effect is he's totally off sugar and chocolate! I'm trying to lose weight and he's trying to maintain as he's lost a few pounds recently. I can't add bread and cheese to his diet too much as it will aggrivate his IBS.

Any suggestions here are welcome

Lots of good suggestions so far. The only thing I can add is for him to get in a couple of snacks during the day. Send protein bars, nuts, etc. with him to work so he can snack on them during the day.

i second putting cheese on stuff
give him creamy based dressings for salads when you use a light vinaigrette
if you eat egg whites at breakfast, give him some with yolks and toast with butter
give him full fat yogurt over fat free
are there any sweet treats he likes that you don't? for example my husband loves mint choc chip ice cream but i would never touch it so i wouldn't be tempted

All of these ideas are great. Also, if he likes to drink calories that can be helpful. Not saying have him down tons of beer and Coca-Cola because of negative health effects! But homemade milkshakes, cocoa made with milk...just milk in general, juices in moderation...those can up daily calories quick as most of us have learned in our pursuit of the opposite goal.

With your husband being so active at work I think it would also be good to put bananas, avocado, etc, in a lot of dishes for him.

My husband is on MFP and watches his calories to lose weight but he's pretty close to his goal weight & very fit. However since he's several inches taller, more physically active, a few years younger, and male, his daily calorie goal is higher than mine. We eat most of the same meals but he adds dry cereal at breakfast, nuts late in the evening, and a double portion of some entrées. If I want 1/2 a banana he'll split it with me...I love that!

Cheese is a good option. You could add a little more fat to his meals - drizzle a little olive oil over vegetables, add avocado slices or nuts to his salads.

He could also have a piece of garlic bread or a roll with butter along side his dinner. If you fix rice, give his a qick stir fry after it's cooked. Maybe add some slivered almonds.