If we read the entire sentence, we see that "him who subjected it" subjected it to futility.
"Subjected" here is the Greek word hypotassō, which means to "arrange" or "set in order" (reference).
"Futility" here is mataios which means "devoid of force", "useless", "vain" (reference)
So, "him who subjected it" is the one who [set it in order] of [vanity].
We can see quite clearly that it was God who set creation in the order of vanity and futility:
Genesis 3:17-19 (NIV)
17 To Adam he said, “Because you listened to your wife and ate fruit from the tree about which I commanded you, ‘You must not eat from it,’
“Cursed is the ground because of you; through painful toil you will eat food from it all the days of your life.
18 It will produce thorns and thistles for you, and you will eat the plants of the field.
19 By the sweat of your brow you will eat your food
until you return to the ground, since from it you were taken;
for dust you are and to dust you will return.”
Furthermore, we can see this supported by a couple of verses in Isaiah:
Isaiah 2:5-6 (NIV)
5 The earth is defiled by its people; they have disobeyed the laws,
violated the statutes and broken the everlasting covenant.
6 Therefore a curse consumes the earth; its people must bear their guilt. Therefore earth’s inhabitants are burned up, and very few are left.
In this passage we see that Adam and his descendants were the ones to violate the covenant (in verse 5), but it's the curse that consumes the earth. This is the same curse we see in Genesis 3 given by God. While humans bear the guilt of the curse, it was God who was the creator of the curse.
While as @Soldarnal answered, both Adam and God were responsible for subjecting the world (setting it in order) in the Garden of Eden, the one who subjected it to futility was God.
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