Usually an ear infection.
Definitely antibiotics plus or minus a steroid depending on the vet. My vet originally prescribed just baytril for Tevy, but we ended up going with baytril & doxycycline when the baytril by itself wasn't working. She was on antibiotics for a whopping 6 weeks!
My vet didn't want to prescribe steroids (can sometimes do more harm than good) but after my prodding she did give me a weeks worth of dex and then you have to wean slowly off of the steroid. Tevy still had a permanent head tilt so I'm not really convinced the steroid helped at all, since I started her on it immediately. The antibiotics are the really important part!
Time is of the essence though, the sooner you get Solis started on antibiotics the faster he will get well and the greater chance of his head tilt going back to normal.
Oh and we all go through hard times, hang in there! It is an unfortunate part of rat ownership. I went through a year where I spent over 2 grand on my rats vet bills and thought no one in my care would ever be healthy. After my chronic respiratory rats all passed away, knock on wood, vet trips have finally become farther and farther between. And I can actually enjoy owning rats again! Your time will come too!