Well then you have some time as Sullivan is little guy. But you should start planning for his future. Keeping an adult Sulcata in our climate is really problematic. (indoor housing can be a disasterous affair).
I work with a turtle rescue and I just tell you this because of all the sad cases we get here on Long Island. People just dumping off Sulcatas (at all times of the year) Just ringing the door bell and running.
If you plan ahead you can avoid being put in a position like this. Whether you decide to accomodate him here (which I have strong feelings about but understand that that is a truly personal decision) or look into a nice sanctuary in a warmer climate for him. Again -- no judgements or preaching here and you do have lots of time. That decision must be yours to make.
Nothing is impossible with planning and $$ to get the job done. but now that you have time on your side it will be a good idea to consider the options and what action you will take when he starts tipping the scales at 50lbs or more.
Good luck with him and please keep us posted on his progress.
Steve