I'd say "he took a course in Italian" In your examples, I'd use "in", or "an Italian course".
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If one studied a language as a language, such as in a linguistics department, it would be on Italian. For example, I took a course on the grammar and syntax of Italian. Therefore, you took a course "on" Italian. If one took a course on any subject in which the language was Italian, such as an English literature course in Italy, it would be in Italian. For example, I took course on English lit