The problem comes from his gf, which is natural.
He does not have the strength, courage, ability, to keep his gf
assured. Not his fault because it is not easy, and it largely
depends on his gf's maturity and confidence.
His gf is afraid of losing him, and sees you as a threat,
therefore jealousy arises. Don't expect her to believe that
there is nothing going on btw you and him. She don't believe means
she don't, and theres nothing anyone can do anything about it.
So leave him, ....... alone. You will cause the two of them to
quarrel.
Don't write him off, just leave him alone.
In the meantime, he is just someone that you used to know.
Singapore is a small place, just move on, and one day you will
surely bump into him.