Why is everybody so negative about this? He's doing something weird, but he is still leaving himself open to scientific evidence, which is better than the average mystical / religious diet recommendation (chinese medicine, hinduism, vegetarianism) which mostly do not accept scientific evidence, and instead do their diets for unassailable moral/mystical reasons (balancing "qi", balancing hot/cold foods, purity obsessions, etc.)

People survive on life rafts eating flying fish and drinking rain water, for months at a time. Most people using soylent will not eat it for all their meals, and they will almost always be surrounded by other food they can eat if they start feeling bad. So I don't think it's a big deal, let him do what he wants!