As discussed under 1 Nephi 17:48, the original text allows the grammatical number to switch for generic pronouns in the same passage. Here in Mormon 9:21, the first pronominal reference to whoso is the singular he (“whatsoever he shall ask the Father”), but the second one is them (“it shall be granted them”). Although it is possible that here them could be a mishearing for him, the plural them is also textually possible. For instance, in the list of examples under 1 Nephi 17:48 there are cases where the pronominal variation involves he and they and also his and their; these shifts in pronoun number are not due to mishearing. In his editing for the 1837 edition, Joseph Smith emended the them to him here in Mormon 9:21, but the critical text will restore the earlier them, the reading of 𝓟 and the 1830 edition (and presumably 𝓞).
Summary: Restore the them in “it shall be granted them”, the reading of the earliest textual sources; although them could be a mishearing for him, there is clear evidence elsewhere in the text for shifting in grammatical number for generic pronouns.