According to the housing legislation of the Russian Federation, the landlord can give (prescribe) to the territory of their living space their children, grandchildren, other relatives only with the consent of all living (registered) in this residential area of people, even those that at the time of registration do not exist. If in fact the grandson living with his grandmother and/or grandfather, and the mother against the registration in the house of the elderly, then you can register the child, and for that you need to collect the necessary documents and submit them to the territorial body of the FMS.
Gather your documents, such as personal passport, the passport of the mother or father of the child and the passport of the child (if available) or birth certificate, a piece of statement (departure) of the child from the previous place of residence, certificate from the place of residence of the parents or two references, if they are registered separately, the house register, a certificate for an apartment, extract from BTI, confirming the right of ownership of the premises or the contract of sale and photocopy, permission of the parents on the registration, certified by a notary and the documents confirming the fact that the child is living with you (for example, student ID if the child attends school in your community, etc.). Attention! If the child is under 14 years of age at the time of registration, it is also necessary to provide military ID or evidence of outstanding military service.
Report to the registration authorities, together with his grandson, if his age has reached 14 years of age, otherwise, to appear in the bodies of the Federal migration service can without a child with all the above documents.
Write a statement modeled on the registration of a grandchild, if you are the owner of the property, in the other case (if you are not the owner) will take your statement and the statement of the owner.
Remember, prescribing grandson to yourself, you give him full right of use of premises along with all the registered persons, and to deprive such a right in the next possible only on the basis prescribed by the housing law or by court order.