Liquid diet to gain weight

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My boyfriend is currently having some dental issues and because of this he is in pain quite a lot and it hurts him to eat. He is also very underweight for his height. I was wondering if people have been on a liquid diet before and if so what types of things did you consume when on said diet? By liquid diet I mean like smoothie diets and soup diets. He is allergic to nuts and gets really bad heart burn when he consumes onions. I figured making my own up and go type drinks for him would be a good idea but if that's all he is consuming before getting his dental work done he is going to get pretty bored pretty quick.