He merely used Aristotle's five causes to expound upon five ideas of his own, IIRC.
I think his actual reasoning was faulty, when looked at in a logical form. I seen a syllogism and it didn't work.
His argument is persuasive, slightly, but not conclusive, and certainly he didn't prove God exists as it couldn't be inferred.
Proof comes from valid inference. Anything about God is hypothetical, and intangeable, and is not evidential. Therefore it is vacuous conjecture to assert than one has proved God. Not that I'm saying Aquinas claimed to have proved God. You have claimed that he did this.
If he actually said he did, then it's impossible that he used reason.