The property coming to the hands of "X" by way of partition of ancentral property would continue to be an ancestral property in his hands, in which all his sons and daughters would have an equal share as co-parceners.
As such, so long as the said property is in the hands of "X" as an undivided property (i.e. not partitioned between the co-parcenars i.e. himself, his sons and daughters), Mr. X cannot dispose of the property treating the same to be his absolute property. Mr. only has an interest in the property, but the share is indefinite.
But, once the said property is partitioned, then whatever share that falls to the share of Mr. X, it will be his separate individual property and he can do whatever he wants with it. He can give it as a gift to his disabled son.
In view of the above, the action of Mr. X in gifting away a portion of the land got by him through partition, in favour of his disabled son is open to challenge by his other sons and daughters.