I'm sorry to hear this and it sounds like you're doing your best. we also relied heavily on smoothies and soups for my dad but I do agree with other posters - see if you can get supplements that can be added to his smoothies and soups for added nutrition and calories.
Have you tried adding avocado to your smoothies? Lots of good fat, extra nutrition and calories and adds a creaminess rather than a strong flavour - you might need to experiment a bit to find the right amount but eg can be added to a fruit or veggie based smoothie.
Soak oats overnight in water/juice/yoghurt/milk (non dairy if preferred) and add to smoothies. Again, increases calories and nutrition. Flaxseeds can be good for the constipation too.
Definitely use prune juice as the liquid. The flavour isn't fab by itself so again, you might need to experiment with the volumes.
Can he tolerate dairy? Full fat greek yoghurt can be a great addition to the smoothies.
Butternut soup can be blended smooth and is quite nutritious. You can bulk it up/add calories by adding sweet potatoes or regular potatoes and/or beans. I have a couple of recipes I made for my dad when he had cancer which I will see if I can upload shortly.
Again, if he can tolerate dairy, a great soup that is, admittedly,y quite low in calories but quite nutritious is:
Saute some chopped onions in olive oil until soft.
Add a bag of spinach and roughly 2 cups of frozen peas (or a small bag of frozen spinach). (you can adjust these amounts - when I'm dieting, I use more spinach, less peas but the peas add additional sweetness).
Pour in some hot stock to wilt the spinach. Once wilted, JUST cover the spinach and peas with the rest of the stock (I use chicken but obviously veggie stock is fine).
Boil for 10 minutes.
Once slightly cooler, use a hand blender to whiz it up. Squeeze in some lemon juice, a grating of nutmeg and some salt and adjust as necessary.
Serve in a bowl with a large tablespoon of greek yoghurt swirled in.