I seem to remember Ongard was around the same age when he first began to spend time with Bong Su in late 2013. By the following year, they were essentially grouped together more often than not and grew quite close. I know Ongard still slept with the cows overnight, but it seems his actual interactions with the herd were limited by that point.
Ongard was very independent though; Roi Yim obviously is much the opposite, so I too am glad Roi Yim's managed to grow his confidence. When I visited a few months ago, he was spending the afternoon with Aiyara and Kulab, so perhaps the new Werribee site has allowed him to explore and be more independent.
I imagine that by the point that the next calves are born, Roi will be spending most (if not all) of his time with Luk Chai. Man Jai certainly had the beneficiary of no additional calves during his time with the matriarchal herd. He was only five though when he was essentially kicked out, so still on the younger side. Had Willow survived, I imagine we would have seen Man Jai transition out much earlier too.