grant (v) 授予

I grant him to get food.
We always say {grant sb. something}.
You can use {grant sb. permission to do something}.
I grant him permission to get some food.

They grant me to buy snack.
{Snack} is a countable noun. Use plural form here.
They grant me permission to buy extra snacks.

He grant me to help other people.
{He} is the third person singular, so add an -s at the end of the verb.
He grants me permission to help other people.

{Grant} is always used for some noble things.
The genie granted me my wishes.
I grant you permission to marry my daughter.