He's been sitting on His throne since 70AD
When the Lord Jesus ascended into heaven He was set down at the right hand of the heavenly throne of God:
"Now of the things which we have spoken this is the sum: We have such an high priest, who is set on the right hand of the throne of the Majesty in the heavens" (Heb.8:1).
However, the Apostle John witnessed a scene set in the heavenly sphere and in the following passage he speaks of a throne which he saw there:
"After this I looked, and, behold, a door was opened in heaven...And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne" (Rev.4:1-2).
The one who sits upon the throne is Almighty God (v. 8) and John saw that that He had a book in his right hand:
"And I saw in the right hand of him that sat on the throne a book written within and on the backside, sealed with seven seals" (Rev.5:1).
Then John saw the Lord Jesus take the book out of the right hand of God:
"And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth. And he came and took the book out of the right hand of him that sat upon the throne" (Rev.5:6-7).
First notice that John saw the Lord Jesus "standing" and therefore at that time He was not sitting on any throne. Besides that, when John saw the Lord Jesus and God together he only saw one throne and it is God who is sitting upon that throne and not the Lord Jesus. And John speaks of both of them in the following verse and he speaks of only God being upon a throne:
"Blessing, and honour, and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever" (Rev.5:13).
If the Davidic throne is in heaven now then why didn't John see it? John had a clear, unobstructed view of the heavenly scene, being able to see the throne of God and also able to see seven lamps of fire burning before God's throne (4:5). But he did not see any throne on the right hand of God's throne. He also saw four beasts at God's throne (Rev.4:6) and he counted twenty four elders around that throne (Rev.4:10) but he did not see a throne at the right hand of that throne.
John didn't see the throne of David and besides that, the Lord Jesus was seen standing and not seated on any throne. But you say that He has been sitting at His throne since 70 AD. Also, at Acts 7:57 Stephen saw Him "standing" at the right hand of God so you are wrong when you say that He has been sitting on His throne since 70AD.