How should I talk to my junior in the office about his habit of coming late to the office?There are 2 ways to handle such people:
Take him out for a coffee. Start by telling him your expectations from him as a senior member of the team. Seek out his aspirations. And raise the issue of his deliverables with respect to his experience. Talk about the fact that other members in the team look up to him for mentorship and that his responsibilities are going to increase. This should ignite some spark in him to make him more productive.
2. Micro management
Hand him responsibilities in front of the team and ask him about his deliverables during sprint meetings. In case he skips sprint meetings, catch him when he is in office and call up a sprint meeting and ask him his deliverables. It might be a bit humiliating for him, but should do the trick.
You can also mix both the ways. But I would recommend you to start by first motivating him and then move to micro management. Avoid humiliating him unless he becomes even more irresponsible and his performance degrades further.
BTW, coming late should not be associated with deliverables. As long as the person delivers, I would not care if he comes to office or not.
Start off by taking him to the tea machine 10-15 mins after he's settled in and try to talk to him about his late coming. Tell him how everyone expects him to perform better than current output as people have high expectations from him owing to him being more exp. That ways you try and give him an indication that he is somewhat under spotlight and he may improve his timings.
If that doesn't help, then during team meetings assign clear cut deliverables to him and others in the team and track them very closely (for everyone including this person so that he does not feel targetted)..