Immigration is no solution at all.
First of all, in the last five years alone, China's births dropped by about 8 million. The total births in all of Southeast Asia per year is only about 11 million. How are you going to get 70% of the childbearing women in Southeast Asia to move to China? You're not. You're talking orders of magnitude a far greater migration than the US/West has welcomed, and the US/West has a higher per capita GDP to attract migrants. Besides, Southeast Asian countries are mostly at replacement level births and in some cases below it already, they need those babies to support their own economies. Japan and South Korea are in even worse shape.
Second of all, even if you get hundreds of millions of new immigrants, if they assimilate into a Chinese culture where a TFR of 1 or lower is the norm, then you're just going to have the same problem again in a few years. China's problem isn't that it's total population is too small but that it's annual birth rate is too low and unstable.
Simply put, there is no substitute for stabilizing its TFR above 2 for China. IMO China has hardly scratched the surface on the pro-natalist measures that should be taken. For instance
* If a married couple under 30 buys a house, they get it 10% paid off. If they have one child within three years, it is 25% paid off, if they have two children within five years, it is 50% paid off, if they have three children within seven years, it is 75% paid off.
* Parents are subject to lower tax rates across the board, greater social security benefits, rent subsidies, and offered free 24 hour childcare. Young people with multiple siblings are offered preferential treatment in entrance to competitive colleges similar to how ethnic minorities receive them.
* Employers who employ a greater percentage of parents within their workforce are given massive tax breaks. The more children their employees have, the more tax breaks and subsidies they get. Further, all parental leave is compensated.
In short, it requires spending money, and a lot of it. Another means is promoting entertainment and TV shows where having multiple children is the norm. Basically, there is a lot you can do. People need to recognize that modern society is completely different than traditional society. At that time, having children was an asset because of child labor and the fact that children stayed with the family and became the family's labor force essentially, then took care of parents in old age. Now society has to transform again to sustain itself.