My dear Rita:
Thanks for wiring this letter to share your problem with me. I am also willing to help you answer this question. I hope the following suggestions of mine can help you.
First of all, you should accept that environment has a great impact on children's language learning. This is not only for children born abroad, but also for many Chinese children who originally lived in China. Many of my Asian and Chinese friends went to Britain and the United States, and their girls followed them to English speaking countries. They lost their Asian culture and became English speaking people. So it's normal for your son Patrick to show his closeness to German and his estrangement from Chinese, because he has been living in this environment. But how to improve multilingualism? Now I have some ideas.
First, start with the family environment. I once interviewed a transnational couple and they told me about their child Elliot. The following is what the mother said:
"I’m sure that family language environment is particularly important. I didn't force Elliot to speak the certain kind of language at first, because I felt that it is better to let nature take its course. So I would speak Chinese with Elliot, but he wanted to speak English with me, and alright, I would also speak English with him. During the age of three to five, we all used English to talk with each other. Because I believed that it didn’t matter as he could always get a chance to contact more with Chinese when he went out. But Later I suddenly found that his English has always been better. Living in China, he even could not say a complete Chinese sentence. Till later, my husband went abroad to work for half a year, and his grandparents came to take care of him. That made a great progress for his Chinese learning. And his Chinese was no longer filled with foreign accent and could be more authentic."
So from this example, I believe that you can find some key point for you to do. It's not enough that only you can speak Chinese with Patrick in your family. You must give him a language environment to learn Chinese. Like Elliot's mother, you can take your parents from China to live with you, or take Patrick to your parents for a period of time, so that he can have a language environment for learning Chinese. Maybe at the beginning, he will be very uncomfortable, but after that, his progress in Chinese will be shocking and fabulous.
Second, put more efforts more in the bilingual or multilingual education together with your husband. Research shows that if parents who are busy with their work or indifferent to their children's language learning, their children tend to become monolingual speakers. In the process of children's language acquisition, family is an important place for their multilingual formation, and foreign parents play an important role in the process. You mentioned in your letter that you and your husband are usually very busy and have no time to take care of the children. I would like to say that parents have a great influence on their children's bilingualism. If you don't have the enough time and efforts to teach Patrick to learn Chinese, he will never be interested in it. You can go to the mall to buy Chinese children's picture books, which contain as many interesting pictures as possible, and then read them with Patrick every night before he goes to bed. Pictures will make children curious. With curiosity, Patrick may be interested in Chinese.
Last but not least, I』d like to say that children's language development has the characteristics of individuality and variability. I have studied the language development of children in seven groups of transnational families. Here are the results of my survey: