I don't think sleeping on their sides is a bad thing either. All of my chins do it from time to time and some more than others. Gideon will even sleep on his back in his hammock with all 4's up in the air...scared the hell out me the first time he did this!! As long as they're eating and drinking well, their poo is normal, they are alert and active (not lethargic), and not overheated, I think it's fine. I also believe (and I could be wrong) that once your chin starts sleeping on their side, it's an indication that they're comfortable enough in their environment to "relax". When I first brought each of my chins home, they always slept sitting up or on all 4's...kinda like they were "ready to run" if they needed to.
It sounds to me like Phoenix has become very comfortable in his home and with you...this is a great thing!