Grandfather can will away the self acquired property to the grandson. even in the ancestral property, only his share can be given to the Grandson by way of will. I am assuming that you have a hindu background while answering your query. with specific details of the property, if mentioned, proper advice can be given.

Further, any property which is in the registered name of the grandfather, by way of WILL or GIFT, it can be transferred to grandson by providing a proper reason and the DEED or WILL be got properly registered to give it an authentication.