My supervisor (in a big German company) has offered me a master's thesis and a PhD position in his company. Before I would like to start the PhD there, I would like to do some other internships to get more experience in different fields and write my master's thesis somewhere else. It's not that I do not like the topics in the company, but I want to find out what it's like to work in different companies and fields. How can I tell him about this without giving him the feeling that I do not like the work and would like to come back probably?
Understand the specifics first: To make an informed decision, you clearly need to understand the specifics of the position you've been offered: Can the start of work be deferred? If yes, how long? Does the company have such openings frequently or is it an unique opportunity? I suggest to ask these questions to your potential supervisor first, before making an announcement that you might later regret.
Honest communication: Tell him exactly what motivates you to defer your potential start of work. There is a non-zero risk that he will still be disappointed, but this risk cannot be avoided, no matter how good your communication skills are. Also, be prepared to negotiate the date when you're ready to start work in his company.