If you don't mind spending a bit on one or two items for the cage, I got this for our current robo and he really loves it. It has a hidey place underneath he likes, he sits on the little shelf and climbs all over it. It's good in a low cage like that one he has now as not to far to fall onto if happened to climb to the roof bars (but robos rarely do). It could help fill up the cage a bit along with some cardboard tubes and hides. There were no fake plants or moss on mine by the way and if there had been I'd have pulled them off!
Amazon.co.uk
Looking at that cage also, I think you would do best to enrich it now and get him settled and happier, before thinking about upgrading him. Then when you do upgrade him everything will be familiar and smell familiar and can just be moved across to help him settle.
He doesn't seem to have a nest? And no doubt that little house ends up needing cleanign out a lot which could stress him. So cardboard hides will give him somewhere to build a safe cosy nest that won't be disturbed. Any house that's open underneath and sat on the substrate stays aired and doesn't get smelly like the enclosed plastic ones.
So suggest adding a bit pile of strips of torn up toilet paper as well - he'll take it to where he wants to build a nest.