I'm so sorry for your loss. I lost my younger brother to suicide 7 years ago, he'd just turned 18.
At the time, there were no signs. But in hindsight, they were all there. Isolating himself, awake all night and sleeping all day, avoiding speaking to anyone.
None of this is your fault, and if someone is determined to take their life then nothing will stop them. It's a decision they've made, and it's like a weight has been lifted. I struggled with anger towards him, how dare he, how selfish. But now I'm out of the grief cycle, I realise that he wasn't selfish. He was so mentally unwell that he was convinced he was doing us a favour, and he was a burden and couldn't go on any more, as he wrote in his note. None of it was true, but depression and desperation took over his mind.
I remember the early days so clearly, I was in autopilot. I couldn't accept or even believe it, I just kept thinking I'd hear from him soon or he's just gone on holiday. It wasn't until the funeral a month later that it slapped me in the face and I had to practically be carried out of the chapel, I couldn't walk, riddled with anxiety and grief and my legs turned to jelly.
Others who'd been through it told me it won't always feel this painful, and I didn't believe them. I didn't see how I could possibly move on or be happy again without my brother. But in time, it will feel less raw. You'll be able to be reminded of him and it make you happy and not burst into tears. Memories of your son, happier times. His interests, his life and so on. You learn to live with it, you never get over it, but the pain settles.
Look after yourself, don't feel pressured to do anything you don't want to do. If you want to surround yourself with people or throw yourself into something to distract you then that's fine. It's also fine to lie in bed and scream and cry. Whatever gets you through the next minute, next hour, next day. Baby steps.
Once again, I'm so sorry you're going through this. It truly is the most painful thing to experience and live with.