What are they basing the diagnosis on - is it just from the high coronavirus titres? if so, that
isnt a definitive diagnosis of FIP, its only confirmation he's been exposed to coronavirus, which is a very common virus, many cats carry it. If your cats have previously been close then its highly likely your other cat has coronavirus antibodies anyway - that doesnt mean he will get FIP. Your other cat can't 'catch' FIP from Rolo, FIP itself isnt infectious, only the coronavirus that
may, rarely, cause FIP. Do look at Dr Addie's site it will put things in perspective.
I think its good that they are treating with antibiotics and steroids, its what Gwynnie was on when they thought it was FIP with her. I have no experience of interferon, but its definitely worth trying and the fact the he is still bright in himself and eating is sounding positive to me.