Gaining

Carbohydrates like pasta,bread and potatoes.Eat constantly in the evenings until bed.Gradually increase portion sizes.Keep a range of savoury and sweet snacks in the house at all times 😊

Don't. Consent is essential. It's lovely that he's agreed to gain some weight for you, but causing him to gain without realising is abusive.

If he's not super active then protein is a good one. Any protein that your body doesn't use to rebuild will ditch some nitrogen and turn straight to fat. Plus it doesn't clog up your arteries all that much! If he is active, then a lot more protein should do it.

And like everybody else says, high fat and sugar foods will work very well. The key is to keep it irregular (fasting for a bit, then stuffing to the brim).

I'll also note that I'm speaking from science, not from experience, so take it all with a grain of salt. And some butter...