He must be repairing his computer now
You don't need two sentences. The above can be interpreted in at least two ways. In written English, meaning is derived from context. In spoken English, it is derived from context plus tone of voice. Here are two hypothetical conversations:
1. Expression of belief
A: Is John repairing the computer?
B: I'm not sure.
A: Has he arrived yet?
B: Yes he went your office 5 minutes ago. He must be repairing the computer now.
A: Good. It's been out of action for two days now!
A: Is the engineer repairing the computer?
B: No he's only just arrived.
A: What?!! He must be repairing the computer NOW!
A: Because the boss has just walked in and he will go mad if he doesn't see the engineer fixing it.