you are assuming that in this verse the one speaking from the throne is Jesus, however, nowhere does the verse directly state that it is Jesus speaking.
From my interlinear, the account reads:
Rev 21:3 'And I heard a loud voice from the throne saying behold, the tabernacle of God is with men, and he will tabernacle with them and they will be his people and He himself will be with them
Vs5 'and said the one sitting on the throne, behold new I make all things and he says write because these words are faithful and true, VS 6 'And he said to me they have come to pass. I am the Alpha and the Omega, the beginning and the end....VS 7 The one overcoming will inherit these things and I will be to him God and he will be to me a son.
When you look at the passage in context, it is the one sitting on the throne who is speaking and saying he will be God to the one who is overcoming. It doesnt state that the one sitting on the throne is Jesus.
If you look back at Revelation 4:2-8, the one sitting on the heavenly throne is receiving worship from the all those who dwell in heaven. He is called Lord God the Almighty. Never is Jesus called 'the Almighty' anywhere in the bible.
Rev 4:2 After these things I immediately came to be in [the power of the] spirit: and, look! a throne was in its position in heaven, and there is one seated upon the throne. 3 And the one seated is, in appearance, like a jasper stone and a precious red-colored stone...8 And as for the four living creatures, each one of them respectively has six wings; round about and underneath they are full of eyes. And they have no rest day and night as they say: “Holy, holy, holy Lord God, the Almighty, the one who was and the one being and the one coming'
And this idea that Jesus is different to The Almighty is borne out from the Apostle Pauls where he goes on to tell us that, when Jesus comes to the end of his thousand-year reign, Jesus, in turn, subjects himself to the One who subjected all things to him. Why? “That God may be all things to everyone.”
1 Cor. 15:28 'But when all things will have been subjected to him, then the Son himself will also subject himself to the One who subjected all things to him, that God may be all things to everyone.' By Jesus subjecting himself to God, it shows that Jesus is not the Almighty...that position is held only by one...the God of the OT named Jehovah. So it is Jehovah who says 'I will be God to him' in the above verse you've quoted.