Nothing. Some bunnies are stand off-ish while other are more social. Acacia is my stand offish bun. It took me over a year to calm her down (spay hormones included) and after I built their large pens that I was able to get inside I made her know I was going to invade her space whenever I wanted. She had her cave-box to hide under when she needed to. Eventually if she wanted her raisen she had to come up on my lap to get it... As demonstrated in the vid I posted in her "New Crib" thread. She can stretch and do whatever she likes, she does not get her treat until I get what I want.
Simon is not a bad bunny, he just doesn't know. Could be from his past could be just because he prefers his Speckle. Some bonded bunnies prefer their furry companions over human anyway. I wouldn't take it personally. Just keep working on him. Maybe stop giving his treat until he stays. IT will be long probably but it can work. I was told to get rid of Kayshie when she was a terror but I calmed her!
Maybe build a pen which you can sit in or bring them to the bathroom or a small room. Give him his space but let him know that you will invade when you want sort of thing.
Keep us updated!
