A lot of parrots will act out sexually (how can I say this in a nice way? omg)... self pleasure? There I think that will work to explain what I mean
- when they reach sexual maturity and cycle through the breeding season. Or- they do it because they discover it and it becomes rather habitual. Most of the time they don't do it to each other. I have never heard of them doing it as a display of dominance... I could be wrong though!
With birds (as far as I know) it's a sexual display. Most of the time they "hump" their perch. You'll see them crouched down on the perch, ummm ... doing it. It's more common in females than males.
Is he really trying to "hump" the baby? Or is he just breast butting him, pushing him off the perch? Does the father have a perch higher up in the cage than the rest? The dominant bird will always perch on the highest branch. It may just be the father asserting himself and trying to get the younger bird to perch lower than he is.
Now - I could be dead wrong here~ that's just my opinion from what I know!
I'd probably go ahead and remove the one that's getting pushed around as soon as you feel he's ready... sometimes the rest tend to follow suit and one bird will end up getting really picked on. I hope that's not the case with your brood
Hope I didn't confuse you! Good Luck