Help please don't know what to expect

My 5 month old son he was born on August 9th 2021 was diagnosed with cerebral palsy. He has an MRI on March 3rd for his brain. He also has to go for a swallow study and go to a nutrition specialist becus he only weighs 13lb 11oz he is in the 1 percentile for that he’s having a hard time gaining weight. His right side is stiff arm leg and even neck. He can move he talks alot but he’s not rolling or even close to it. His head is like a bobble head because it’s tremors and his arms and legs do too. He will not stand on his feet at all he just holds them in the air. When on his tummy his feet are up never touch the ground. I’m just really nervous for the future and want to know more about what to expect. He has 3 older siblings with zero illnesses. I just don’t know what to do or how to tell or even when to tell his siblings. We don’t know what type he has yet or how bad it will be. But any and all thoughts a prayers and advice would be amazing. I will be watching for this to post and reply to any question anyone has. Thank you.