In Hebrew culture any male descendent of David no matter how distant can be referred to as the son of David. In Matthew's genealogy David is called the son of Abraham and Jesus the Son of David.
As to Thomas' exclamation:
John 20:24-29
24 But Thomas, one of the twelve, called Didymus, was not with them when Jesus came. 25 The other disciples therefore were saying to him, "We have seen the Lord!" But he said to them, "Unless I shall see in His hands the imprint of the nails, and put my finger into the place of the nails, and put my hand into His side, I will not believe."
26 And after eight days again His disciples were inside, and Thomas with them. Jesus came, the doors having been shut, and stood in their midst, and said, "Peace be with you." 27 Then He said to Thomas, "Reach here your finger, and see My hands; and reach here your hand, and put it into My side; and be not unbelieving, but believing." 28 Thomas answered and said to Him, "My Lord and my God!" 29 Jesus said to him, "Because you have seen Me, have you believed? Blessed are they who did not see, and yet believed." NAS
If Thomas has understood the prophecies why did he have to see the Lord enter a locked room before he believed? Your explanation denies what is recorded in John. Also to base a doctrine on just John 20:28 while ignoring the overwhelming Biblical evidence that Jesus did not physically preexist Abraham, and is not God is a bit ludicrous. After all, you take exception with the cults who base their theology on more than just one verse and then appeal to John 20:28 and Matthew 28:19 to support the Trinity?
Now, this is the last time I will post this warning. This thread deals with the Messianic prophecies not the NT. I have given you links to the threads dealing with the NT and the Trinity. Your next post that goes off topic will be deleted.