What Appeton Weight Gain provide is calorie in liquid form, which is the MAIN reason why most underweight people are underweight. They consume less calorie than their body burn hence reason why they are skinny.

For the Appeton Weight Gain (or other similar weight gain product) to work, they need to make sure that Calorie Consume > Calorie Burn by Body.

What you can suggest to your cousin is a fix meal plan for a week.
Put him for a 2000kcal diet plan consist of 5 meals a day. 3 solid meals (1600kcal) + 1 times Appeton Weight gain (400kcal). If he unable to gain weight or losing a lot of weight, bump the calorie intake to 2300kcal. Rinse and repeat until you get desire result.

Appeton Weight Gain is not a magic powder, nor I recommend people to consume it but understanding how small is the appetite for skinny people, it's understandable.

What's the difference between gain mass and gain muscle and why are those conflicting with each other?

You are not answering my questions and I'm confused by your answer atm.

That's a poor attitude to ask someone for an advice.

Don't stop going to gym, what you need to do is just increase your calorie intake (eg if you take one shake per day, increase to two shake, or eat more food than you currently do).

Problem is that you have very minor knowledge on what you are asking and I tried to help you fixed that. If you failed to understand the basic terms, I'm very sure you are not gonna understand what I'm gonna explain to you.

But well, it's ok. I'm not the one who unable to gain mass and muscle in the same time. Yet again, what's the difference between muscle and mass?

It's ok bro. People who genuinely required advices can just PM me and I'll be glad to answer them when I have time. Prolly my method dont suit him but he is fine to ask someone else, hopefully with not that kind of attitude.
Musclemaniaclub, egonutritions, musclesup,  or any supplement shops in Malaysia which easily can be google.

This is what happens noobs or skinny guy wants to build muscle really fast. Bruh do you really think you can build amount of muscle for the last 1 month? Then fuck no. Your diet affects your gains not your serious mass.

Guesstimate via myfitnesspal. +/- 10% accuracy.