For a good ripe avocado...it should feel abit soft when you touch it. If you're not sure, try to store it at room temperature for a couple days (just like bananas)...you can tell the difference ga la! The texture of a ripe avocado should be very soft and creamy. You can easily mash it by a fort ga. My baby loves avocado and it is a very nutritious food for babies! Try to google about the nutrition content of a avocado. You can mix it with banana or yogurt or even spread it on a toast.
I actually use the full cream natural yogurt which means there is no added sugar and it doesn't taste sweet at ll. I would mix it with pear or apple or plum puree and avocado ar. The brand is called Pauls All Natural Yogurt Original but I'm living in Australia. I'm not sure if they have this brand in Hong Kong but it doesn't matter which brand it is as long as it is full cream and natural (no added sugar).
Another suggesstion for putting on some weight is to add another meal..i.e. 3 meals a day (breakfast, lunch and dinner) + snack. I know it would be hard to fit them all into your schedule but try to include 3 meals a day.