Glad he's using the loos! Our last Syrian was the same with overhead space and his first cage was much lower. He always liked to sit "under" something and would "rush" across to the nearest cover. I suppose it is instinct as they are prey animals who expect to be got at from above. Although some are bolder and more maverick than others and seem less bothered.
It can be a fiddle! You hang something for overhead cover and then they try and get into it! So then you have to look at how they can access it easily/safely and not land on anything that could hurt them if they jump out. It's not that difficult though, just a fiddle sometimes!
I'm sure he'll appreciate keeping the tunnel! If it's any consolation both our syrians ignored the carrot house in the cage, but enjoyed sitting in and chewing when out of the cage.