Certainly there are ways which can be tried to help you gain weight. It would have been helpful if you would have posted your investigation report along. Few dietary modification and exercise can be added to gain a healthy weight even in connective tissue disorder like Marfan's.

For gaining weight follow three main things like healthy diet, exercise and walk  early morning and evening which increase your metabolism. don't skip  your breakfast which increase your appetite. take High calorie  and the rich protein in your diet. and good fat in your diet . avoid oily and fried food, fast food  . you need a complete diet plan under guidance of dietitian ,weekly basis your plan will be changed. contact me for customised diet plan through practo within   one month you will get good result  definitely. you can check my feedback and connect me for  customised diet plan

Please ask me directly for personalized diet plan for you.

Yes absolutely.. Balanced meals on time(3main meal & 2 snack pattern) with 9hrs sound sleep will help. Go for 4 fitness components tests , try to improve your fitness level and incorporate active lifestyle. Only protein powder(without prescription)cannot help, you need track sleep , regular exercise ,stress factor, nutritious meal and fitness level will surely help

Pls consult directly through practo app for proper management

Gud morning pls try to follow a healthy balanced diet with some forms of muscle gaining exercises,proper sleep in night minimum 6-7hrs ,adequate water intake n stress management is also important for maintaining a healthy body weight.