My son had the surgery almost a month ago. He is a big man 6'6' /500 lbs and he is very active. He hasn't been feeling well and he thinks it is because he is not getting enough calories. Any suggestions?

All I know from my surgeon' talk is that he needs at least 100 grams of protein. (Men and people with a higher starting weight need at least that much protein.)

Yes, he is getting over 120 grams of protein. His issue is getting enough calories

How many calories is he getting? I found that 95% of the time when I feel run down, especially in the beginning, it wasn't lack of food, it was lack of water. Try increasing water and tracking it consistently. Also if he is extremely low carb, he may see if adding a bit of carbs boosts things. If neither of those work, have him talk to his surgeon and ask to have his labs checked. Best wishes that he feels better.