It sounds like Butter is “coming into his own” a bit early… lol. Testicles usually descend between 3.5 and 4 months old and that is when you see hormonal behavior begin. Having access to a beautiful young lady may have sped that up a bit tho.
She is too young right now to get pregnant, but I would be prepared with separate housing within the next couple weeks. They should be separated by the time she is 12 weeks old.
Assuming HE gets neutered ON his 16 week birthday, that is still 4.5 weeks away. By that time SHE will be 14 weeks. Still, probably a bit too young to get pregnant…
BUT…
Males retain viable sperm for 4 weeks (sometimes 6 weeks) AFTER THE NEUTER!!! A neutered male can still impregnate a female rabbit during that time!
She will be 18-20 weeks before he has “clearance” to be with her again. By that time, her hormones will have “dropped” and it’s likely she won’t accept him until after SHE has been spayed (at about 6 months old).
So long story short, separate them when she is 11-12 weeks old. Get everyone neutered and spayed at the appropriate time. Begin bonding sessions once your girl is recovered a full month from surgery and your boy has been neutered a few months.