Hi I'm 6'1" and I was down below 120lbs. It took some time and getting my pouchitis under control was critical but with a very high calorie diet (about 4000 calories or more daily) and a highly reduced physical demand on my body and I am finally back to 180lbs. I also have a high metabolism and always struggled to gain weight. Cannabis is the only way I can eat as much as I do. Not everyone can stop doing a physical job but I needed to.
Good luck and definitely try and find out what you need your calorie intake needs to be, I also thought I was eating enough but you might be surprised.