Bilinguals have split personalities. However, this may be because they are acting according to different cultural norms when speaking each of their languages. When speaking English, they assume the cultural role expected of them in English-speaking society. This is different than the cultural role expected of them in German-speaking society, which they assume when speaking German. The change in language cues a change in cultural expectations.
Bilingualism is a charming exception, but monolingualism is of course the rule.
But it is very reasonable to guess that over half the world's population is bilingual. Most of those who will read this live in countries where monolingualism is the rule, but are seeing a very unrepresentative sample of the world. More information can be found on the Practical Help page. You'll never manage to make him bilingual now. However, people can learn valuable language skills at any age.