Well, what you could do is eat before you go to bed, but it also depends on how high of a metabolism you have. Do you exercise a lot? If you do, I suggest putting a hault on the exercise.

dont do exercise, havent for long time.

Drink alot of water.

Weight gain is something that isn't easy to do and you probably know this already. The key to weight gain is to do everything BIG. You have to eat big, to lift big, to get big. Say that over and over again in your head until you fully understand it. A lot of people think weightlifting is the key to and I won't argue, it is an extremely important part. BUT, there is another thing that is just as important when it comes to how to gain weight, and that is your diet.
So, to put it as simply as possible, there are 5 simple steps to how to gain weight, and here they are:

1) Count how many calories you eat in a normal day. Don't change anything, just eat like you normally would and count how many calories you consumed. This is an extremely important step, so try to be as exact as possible. Also, weigh yourself.

2) Starting the day after you counted calories, eat 500 calories MORE then you normally do. So, lets pretend that the day you counted calories you counted 2000. For the rest of the week, you would now eat 2500 calories a day. Instead of eating
3 big meals a day or eating all day all the time, spread those calories out over 5-6 smaller meals. Eat one meal every 2 and a half to 3 hours. To get big, you have to eat big! Remember that.

3) Weightlifting! Get in the gym and lift! This is another important step to how to gain weight, so make sure you are doing it correctly.

4) At the end of that week, weigh yourself. You'll notice you're gaining just after one week! Now, don't expect to see a 10lb increase. Gaining anymore then 1 or 2 pounds a week is unhealthy and means you're putting on way to much fat. So look for 1 or 2 pound gains at the end of the week. Don’t sounds like much? You can be gaining 5-8 pounds a month!

5) Here’s an important one. At some point, you will stop seeing weight gain. At this point, you will have to eat even more. So, when you stop gaining for at least 2 weeks, it means it is time to start eating an extra 250 calories a day. Every time you see you haven't gained weight for at least 2 weeks, add an extra 250 calories.

Including these tips you may take some supplements also which helps you in weight gaining.