Porridge with smooth peanut butter stirred into it, with mashed banana and or jam for breakfast
Lunch may have to be soup, but it doesn't have to be watery. Make a really thick lentil and root veg soup with spuds, carrots, etc, and then puree it with a stick blender. Smooth stew, if you like. Chuck in ham too as you cook and before you blend. It might look disgusting but with the right spices for him, it'll do the job.
Or a slightly thinner but still pulse heavy soup that he can put bread in (crustless white if need be) to soak and turn into mush. Baby soup is one of my favourite lunches.
Yogurts, milkshakes and smoothies, especially with protein powders added.
Ice cream. Okay, not the healthiest, but it's high calorie and will be easy to eat/slurp.
Risotto, really well cooked, or very overcooked small pasta like orzo? Pesto and extra cheese for the sauce.
Mashed potato, made extra creamy with more milk than normal, with grated cheese mixed in.
Does he like fish? Fish can be cooked very soft and flaked into mash.