If Fortisip tastes even remotely like the three different brands we can buy or get on prescription in Denmark, your husband will not like them (they are all yoghurt based and taste revolting) and will refuse to drink more than one or two a day.
So get in touch with a hospital dietrician at once and ask what else she recommends.
You can buy yoghurt and add full-fat cream to it, cocao etc. if your husband can eat dairy products.
Otherwise home made chicken, beef, or vegetable soups, strained carefully are a possiblity, but you need advice on judging how many calories a portion contains.
Mashed potatoes are a possiblity if your husband can eat any solid food, preferably with mince. Mashed carrots, turnip, etc are also possiblities. So is rice pudding or any other kind of milk pudding.
My husband could force himself to drink the coffee, vanilla and strawberry flavoured ones, but as they all have a metalic taste (and tasted them myself to make sure it was not his taste buds that were affected, there was no way I could get him to drink four to six a day.
If you have a Victorian or Edwardian cookery book in the house, it will have a chapter on invalid food, which you may find helpful.
But you do need professional advice as to what your husband may and can eat and drink and unless you are a trained chef yourself advice about the amount of calories in anything you make.