First of all, Bart Ehrman does not “believe” anything. He is an atheist and an enemy of Christianity. He is a top scholar, but he comes from an extremely biased perspective in approaching the Bible and church history. This bias comes from the fact that he brings a presupposition to the study of scripture. The presupposition is that nothing in scripture can be supernaturally inspired. It just so happens that Ehrman is wrong. When we bring a correct presupposition to the Bible, it is likely our conclusion will be correct, but when we bring a flatly untrue presupposition to our study of the Bible, it will cause us to grossly misinterpret. This is the case with Ehrman. Because he has dismisses the idea of the supernatural and because he has dismissed the idea of biblical inspiration. Therefore, if you read Ehrman (who, like I said, is a top scholar), you need to read him with a very large grain of salt.
Jesus is the lamb of God “slain from the foundation of the world.” (Revelation 5:9). What this tells me is that from God’s perspective, Jesus already was what he, technically, was not yet. That Jesus is a lamb became true when his life was offered, yet he was lamb long before that, as God views things from outside of time. In other words, what he became at one point it time he always was, at least from God’s perspective.
That Jesus was not the Son of God at one time is disproved by many passages in the Bible. In John 3:16 Jesus is declared to be the Son of God well before his resurrection. The same is true of Matthew 14:33 where the apostles declared that Jesus is the Son of God. Jesus did not deny their assertion. They did not say “You will be the Son of God. They said “You are the Son of God.” Jesus called himself Son in John 5:21 and in many other passages in John. Also, in Psalm 2:7 it is said about the Messiah “you are my Son; today I have become your father.” Jesus was declared to be the Son of God many ways–by his miracles, by his incarnation and, also, by his resurrection. This is the meaning of the Romans passage. The overall message about Jesus tells us that the meaning of Romans 1:4 is that his sonship was confirmed by (not at) the resurrection. Ehrman should know this. I get the impression that he gets a kick out of making trouble for Christians by saying things that he knows full well are not even true. I cannot prove that this is what he is doing, but his irresponsible abuse of scripture leads me in that direction.
The same can be said for Acts 13:33. It is true that in this passage Paul is talking about the resurrection, and that he uses Psalms 2:7, but Paul does not declare in this passage that Jesus was not Son of God before the resurrection. This is obvious, because Jesus was Son of God long before that, as Paul was well aware. Again, the meaning is that he was “declared” (ie publicly identified) as Son of God by his resurrection. But then again, he was declared (proven to be) Son of God on many other occasions as well, as Jesus said, as the apostles declared, and as the church taught from the beginning. He was declared Son by his incarnation, by his baptism, by his miracles and by many other things. This would include his resurrection, but the apostles recognized it long before his resurrection and we have passages which prove this.
What I can say with absolute certainty is that there is no evidence to support his thesis. In other words, there is no evidence from early church writings that there was a time when the church did not believe Jesus was the Son of God. This is an evidence-free statement which can be rejected as simply not true at all.
When was Jesus begotten? Unlike statements about the sonship of Jesus, where we have a LOT of evidence, we have very little to go on with his being “begotten” This is a very abstract theological discussion with few if any practical implications that I know of. What does the word “begotten” even mean? We have to go to the Greek and do some speculative theology here. The Greek word sometimes translated as begotten is in John 3:16. He is the monogeneis. He is the uniquely made one. What does this mean? Hundreds of theologians have torn this word apart and still the meaning is abstract and not completely clear. With difficult passages such as this, the rule of interpretation is to first ask what it cannot mean because of other, clear passages of scripture. What it cannot mean is that Jesus is a created being (because of John 8:58 and a zillion other scriptures). What it cannot mean is that there was a time when he was not the Son of God and then after his resurrection he became the Son of God.
I will be honest with you, I am not sure myself exactly what it means when John tells us that Jesus, the Son of God, the Messiah is monogeneis. Perhaps it means he is the unique one. Perhaps it means he is the only one. What I can say is that this is what Jesus IS, not what he became. I will let you chew on that one. However, what I can say for sure is that Ehrman is flat wrong, both because of biblical statements and because there is literally not a shred of evidence to support his contention.
I hope this helps.