The best way is by spending time with him. Sit him down and talk to him about your office. Share details about your routine and you can even share how you helped solve an issue in office to keep him engaged. Similarly, ask him what he did differently in school. You can turn this into a game. But in the long run, if you work away from home and rarely see your kid, it’s best to spend as much as you can with him over the weekends. Take him out for walks and play games with him. Slowly but gradually you will notice him opening up to you and sharing his school activities. Good luck!
You should come up with this game you tell a story and then your kid has to tell his and this activity basically is describing your day/week and what all made you happy and what made you sad. Can be done daily even when you are away and on mobile. So basically you need to make him feel as if your his friend and both can share everything with one another.
Spend time with kid and it will take some time to gain confidence with you to share things which is not happeing right now. Break down the things into small parts try to have more conversion it might not need to be very long it can be quick chats but frequent Spend quality time with them, try to learn the way they behave and why Speak to them as friend
Try to be his friend. Tell him/her your school day stories and eventually he will share his school related stories too.
Talk to him face to face while you are with him is a good way to build up a relationship with your kid