This is normal: babies have internal mechanisms to shield them from over-stimulation. Sometimes your 9-month-old son might get overwhelmed when he is in the midst of all the activity and be very content to observe what is going on around him. He probably feels he is part of the action just by watching it. Observing is often just as enjoyable as being part of the action, similar to when you watch an exciting movie or a sporting event, especially when there is a lot of action.
You might notice that when your 9-month-old son has time to play by himself, without all of the activity of his older siblings going on around him, he will have more confidence in his abilities and more of an interest in his toys. He might even crawl to get to his toys or pull himself up to standing and “cruise” around holding onto furniture. Encourage your older children to take turns playing with their baby brother one-on-one every once in a while, so they can get to know him better and learn more about what he finds interesting. He’ll get to know them better, too, and form a bond. I’m sure you’ll find that eventually your little one will blend in, join the action, and soon be the center of attention. Be sure to remind your older children to keep their own toys out of reach of their baby brother, especially things with small parts or elements that are not appropriate or safe for him.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.