I was on a strict liquid diet last year for a while and the protein shakes I was drinking were great at filling you up & keeping you full plus you can get creative with them & add fruit (bananas, berries etc) or change what you make them with (milk, water etc). The brand I liked are called: GNC Properformance Lean Muscle Meal. They come in chocolate, vanilla & strawberry. (The ones where you buy a box of packets that you make have 40 grams of protein each - so you never feel hungry & your hair won't fall out...the pre-made shakes only have 20ish grams of protein.)
Alternatively, you can look at blending some of his favourite soups (and then straining them to make sure they're smooth...).
Best of luck with this - what a terrible injury!