I believe you are dealing with a poltergeist. In these cases, the entity attaches themselves to a person, usually a child. Poltergeist themselves cause havoc, usually by throwing objects but also using voice.
An exorcism might work although it usually goes away naturally. I would honestly try to communicate with the spirit and ask why it's there, what does it want, will it leave etc.
Apologies, but I must disagree with the first reply to your current spiritual dilemma. If the spirit has indeed attached itself to the child and you are 100% certain it is not connected to the house or a house, you most likely have either a malevolent ghost who doesn't want to move on or cant, or it is a demon, which in this case is the most probable. Demons like attaching themselves to children due to a child's sensitivity to the paranormal & their purity of heart. Makes it even more messed up, I know. Whatever you do, please don't attempt to contact it if you find it to be a demon. This will open a door to things that have no business being here & it is not easily shut. The author of the first post was correct about the exorcism, it will most definitely solve this problem if it is demonic in nature. Finally, I have never heard of a spirit attaching itself to someone and then just leaving on its own. I could be wrong but that doesn't sound right. Please feel free to contact me with any question you may have & I will do my utmost to help.
Shadow, did any of this happen before the baby was born or has it been building in intensity since the birth? Were there any problems with the birth or anything that caused extra anxiety within the family? How does the child react to the activity or the entity? Does he communicate with it? Do the incidents of throwing or opening doors ever follow a time the child was punished or got in trouble?Some of the activity does sound like a poltergeist but it also describes other entities, but it does seem to be attracted to the baby or trying to get to the mother thru the baby. I agree that you shouldn't try to deal with it on your own. I wish I could be of more help. I can do some research if you can answer my earlier questions. Please be safe and may God be with you all.
Visit Creepy Hollows dot com and do a search for their Dissolve The Attachment Of Any Unbound Spirit Or Unbound Entity service. There are two testimonials there that attest to the service provided. I've ordered spells/spirits from the company and they're legitimate. Their services are usually $50 or less.
Creepy Hollows is well known among witchcraft and occult groups to not be a good place to shop. They're among the names re-selling cheap eBay jewelry as haunted, and doing any number of things which are absolutely unverifiable.
I would recommend against any paid service, or any product claiming to have any sort of spirit attached to it.