Fighting Guinea pigs separated but now one is doing better than the other?

2 months ago i adopted two guinea pigs - Jack and Victor. They were apparently paired and had been together since birth and were said to be about three months old. Initially it seemed pretty clear that Jack was dominant. Victor was terrified of everything but I put this down to them being new and settling in. However, this continued to get worse and when in the run victor would squeal and jump out. It became clear that Jack was bullying victor so I separated them as they were very close to fighting. The vet recommended either neutering which wasnt guaranteed to work or seperation, so I decided to separate them. Since the separation Victor is like a new guinea pig and seems to be doing really well.
Jack on the other hand, not so much. Since being on his own, he hides most of the time. I added a run to the cage and it was 2 and a half days before he even went in it. He is still eating etc but not as much as before.

Jack and Victor have separate runs but they are joined as I still wanted them to be able to socialise but so that Victor had his own space, which he appears to enjoy. When jack eventually came out and realised Victor was there he began squealing, biting at the bars and trying to lift/get under the barrier (c&c cubes). Victor walked over, sniffed him and went away again, still seeming quite happy.

My question is what do I do with Jack? will he get used to seeing Victor but not being able to get to him or do they need to be completely separate? Will he cope on his own? I'm not keen to reintroduce them as Victor seems so calm now. He comes up to the cage door when you come in the room instead of running away at every noise, he sleeps outside of his house now and generally seems happier. This happened within the first few hours of being away from jack, and he's never tried to escape his run since.

Any advice would be much appreciated.