Trees that are grown from seed (and thus are growing on their own roots) have to reach a bigger size before they begin fruiting.
That is one of the main reasons these trees are usually not grown from seed.
Grafting a tree onto rootstock limits growth, and thus diverts the tree's energy into fruit production, earlier on in the tree's lifespan.
If you try to grow a pomelo tree on its own roots, it could take a long time before it begins fruiting, and that tree could grow to be very big. In their natural state in the tropics, pomelo trees can grow to be 45 feet high.
My grandparents used to have a big orange tree that must have been over 15 feet high.
You will one day have fruit, but it might be many years.
If I had to guess, I would guess you might have to wait 5 to 8 years before that seed-grown pomelo tree, of the size already shown in that picture, begins producing fruit. Pomelo trees also take a bit longer than other varieties of citrus.
One last important point, the issue could be pollination. Unlike grapefruit, most pomelo varieties are self-incompatible, and there should be some other different variety around, if you want fruit. (This can even be accomplished by grafting in a different variety into the same tree, either a different pomelo variety or other citrus)
If you're seeing flowers, but no fruit, the issue could be pollination.
However, I think I can see some other citrus tree in the background, in that picture, so maybe that is not the issue.