The law of purity means being clean, as stipulated by religion.
Islam states that a Muslim should be clean when praying.
Your boyfriend is physically disabled.
He might not be able to pray.
If he can pray, then maybe he can shave himself.
If he is religious, he can abide by the rules.
He can decide that for himself.
He can also decide if he wants to be shaved.
But he is not a bad Muslim if he does not shave.
He might find it unpleasant to be shaved by his mother.
He can decide who shaves him.
Your boyfriend has personal rights.
That means for example that he can make decisions about his own body.
And that nobody can force him to do something unpleasant.
Not even because of his disability.