What we cannot conclude from a formal analysis of David Hockney's painting is that the man in the red jacket is the artist's ex-lover.
There is no way we can come to this conclusion - in the painting, all we can see is a person diving in a swimming pool, and a man in a red jacket standing next to the pool, looking at the person who is swimming. There are no other details available that could make us believe this man has anything to do with the artist. I've attached the painting for you to see.