If a person is stubborn in his statement? [duplicate]

It seems you really want to use the word statement in your explanation. In that case I would say:

I have explained the matter to him, but he is stubbornly clinging to his statement.

Stubbornly is defined by Cambridge as meaning:

in a way that shows you are determined to do what you want and refuse to do anything else.

• The president is stubbornly clinging to policies that need changing.

However, there are plenty of other ways you could express someone's obstinacy. For example, this related question has among others

He dug his heels in.

I also came across

to have an anti-everything, nay-saying attitude (lifehack and freedict)

As an aside maybe only slightly related: As I was looking for other expressions I found the humorous:

Mary Mary quite contrary, how does your garden grow (line from nursery rhyme).