the answer is fat, as it keeps the volume low and the calories high.
my husband also has a very high metabolism, and the answer for him is fat. and bone broths. he has olive oil, coconut oil, butter, bacon grease (reserved), duck fat, and then also the fats that are just in foods like meats, nuts, seeds, avocados, and then just whatever else is out there that has fat in it naturally.
the largest volume of his diet is actually veggies -- which we usually steam in bone broth and then he drinks the bone broth as well (so he gets three cups a day) -- and then a bit of meat and then fat.
and he's satiated and able to maintain muscle (he's also a small body type, but no other diseases or issues) as he likes to lift weights.