Sorry if this is long! Going to try to give all relevant details so you guys can give some relevant help – especially if you’ve experienced something similar.
So had first bunny for almost a year, lovely girl, not much used to other bunnies as has been alone since she was separated from her litter. She’s 2 and a half ish. Spayed. Loves her food and loves strokes.
Got a boy in late August, smaller than her as a mini breed but also younger. He is now 6 months. They live in pens right beside each other so can see each other through wire and often lie next to each other through the wire. He was neutered over a month ago and is perfectly recovered from it. Quite excitable and a bit jumpy.
SO.. started bonding them in a neutral, small, clear territory today. They had about 15/20 minutes together. At first she did a bit of looking around and he followed her everywhere, sniffing her bum and her as a whole. Then he started sticking his face and body underneath hers (she is about double the size of him). It wasn’t like he was laying his head down to get groomed – he might have been scared and felt comfortable with her and was trying to hide? So he kept doing this and she didn’t seem to mind but was moving around a bit looking at the walls. Then they seemed to kind of cuddle a bit… Resting their heads on each other (didn’t seem like trying to get the other to groom them). He kept sticking his little face under her belly and just sitting like that like a baby or something, at that point it seemed like he saw her as a mother figure.
This hasn’t been mentioned on any bunny forums as far as I can see. Did a bit of sniffing each others faces.
So then he humped her a bit – which I allowed to happen before stopping him after about five- ten seconds. He did this a few times. 
She just sat kind of hunched – breathing heavily. Seemed a little scared and didn’t move for a while. After this did some more sniffing. Then as he was resting his face on her face, he started to groom himself… on top of her? In the sort of position where he was almost grooming her but not.
Anyway sorry not sure how to act on the next bunny bonding session. Also not sure about that behaviour from the boy – anyone got any ideas?