Gaining

Any kind of weight gainer supplement. Items you'd get from a nutrition store. That might help.

Liquid calories seems like the most straightforward solution as you can drink as much as you can manage as often as possible. Make a huge jug of something and keep it in the fridge. Here are some ideas: livestrong.com/article/438117-weight-gainer-shakes-with-about-4-000-calories/