I am a little worried about my teenage kid. How can I develop healthy habits that keep him away from the mobile phone screen?
Oh wow, I totally get this struggle 
What actually worked for us was creating phone-free zones in the house. Kitchen table during meals, living room after 8 pm, that kind of thing. At first, my son acted like we were torturing him, but honestly, after about two weeks, it just became normal. We started playing board games again, can you believe it? Also, I noticed he sleeps way better now that his phone charges in the kitchen overnight instead of next to his bed. The trick is you have to stick with it even when they complain, because they will definitely complain at first.
Jenny is so right about the consistency thing. I would add that it really helps to sit down with your teen and actually talk about why you are worried. Not in a lecture way, more like a real conversation. When I did this with my daughter, she admitted she felt like she was missing out if she was not constantly checking her phone. That opened up a whole discussion about social media and FOMO that we never had before. Understanding what is really going on in their head makes it easier to find solutions together. 
Been coaching high school kids for years, and the ones who struggle least with screen addiction are the ones involved in sports or activities. Not saying your kid needs to be an athlete, but having something they are genuinely excited about makes a huge difference. Music lessons, art classes, volunteering, part-time jobs, whatever interests them. When they have real-world stuff they care about, the phone naturally becomes less important. Also, tired kids use phones less. Sounds simple, but it is true. Get them moving and active during the day.
Can I just say that I was totally part of the problem? 
I was telling my daughter to get off her phone while I was scrolling through Facebook myself. We ended up making a pact where we both had to follow the same rules. No phones during dinner, no phones after 9 pm on school nights, stuff like that. Having to follow my own rules made me way more aware of my own habits. Plus, she could not accuse me of being a hypocrite anymore, which was happening a lot before.
Here is something that worked surprisingly well for my family. We got a basket, and everyone puts their phone in it when they get home. You can take it out if you need it for something specific, but the default is that it lives in the basket. This one simple change cut our household screen time by probably half.
No phones wandering into bedrooms, no mindless scrolling on the couch. It felt weird for maybe a week, and then it just became what we do. Sometimes the simplest solutions are the best ones.
I think it helps to teach them about screen time tracking, too. Most phones have built-in features that show you how much time you spend on each app. When my son actually saw he was spending 5 hours a day on TikTok, it shocked him. Like he had no idea it was that much. We set up app limits together after that conversation. He picked the limits himself, which made him more likely to stick to them. Giving them some control over the solution really matters. 
Get them outside, seriously! My nephew was completely absorbed in his phone until we started going fishing every weekend. No big discussion about phone usage, just quality time doing something totally different. Now he actually asks to go camping and hiking. He still uses his phone plenty, but it does not run his life anymore.
Find something your kid might enjoy outdoors. Could be anything. Skateboarding, photography walks, geocaching, and bird watching. Anything that gets them engaged with the real world around them instead of the digital one.
Something I always tell other parents is that you cannot just take something away without offering alternatives. Phones meet real needs for teens. Social connection, entertainment, and escape from boredom or stress. Figure out what the phone is meeting, then help them find healthier ways to meet that same need.
If they are using it to connect with friends, maybe encourage more in-person hangouts or group activities. If it is about entertainment, introduce them to hobbies they might actually enjoy. The goal is not to eliminate technology; that is unrealistic. The goal is balance and making sure the phone is a tool they use, not something that uses them.
Look, I am just gonna be honest with you here 
Your teen is probably going to push back no matter what limits you set. That is normal. My daughter complained for a solid three weeks when we started our new phone rules. Lots of eye rolling and dramatic sighs. But I stuck with it, and guess what, she eventually adapted. Her grades went up, she started reading again, and she hangs out with her friends in person more often. Last week, she actually thanked me for it, which almost made me fall over.
Stay consistent, do not cave when they get moody, and remember that you are the parent. They might not like the rules now, but they will appreciate them later. Trust the process. 