Lmao, no but really, I have the same problem. I would say:
Eat late at night
Drink milky drinks inbetween meals
Do not over-exert yourself
Train with weight for added bulk
Eat plenty of carbs and avoid cardiovascular activities
Try iron supplements, apparent;y they increase your apitite.
That's all I can think of, don't take my word for it though, generally do the opposite of what slimmers are advise to do.
To be honest, none of this may make a difference, some of us have a naturally high metabolism or a small frame and/or slim body, you may only put on weight with age. Good luck!