When you say he hasn't matured, do you mean in terms of his genitals, or just generally in his growth and stature? What breed is he?
At 28 weeks his bits should be pretty obvious, even if he hoiks them back up again sometimes, I'd expect you to be able to see the sacs by now underneath. There's some guidance on sexing here:
General how to:
http://www.rabbitnetwork.org/articles/sexing.shtml
Boy pics:
http://www.rabbitnetwork.org/articles/sexingboys.shtml
Girl pics:
http://www.rabbitnetwork.org/articles/sexinggirls.shtml
Is it possible that he's actually a girl: has it been confirmed that he definitely is a boy? Is there a penis pops out when you press gently near his bits, even if there aren't any testicles obvious?
Or they can suffer from undescended testicles. If this is the case, it's best to get him castrated at a competent vet. Undescended testicles is thought to increase their risk of testicular cancer, testicles are not meant to stay in the body, the temperature is too high for them.
I would pop him along to the vet for them to have a good look and see what they think. Has he had his vaccinations? At 28 weeks it could be a good time for a general health check and any vaccinations if he needs them