[lynnedit]

All good advice.
Poor guy, you don't want to make too many sudden changes. As long as the heat mat does not get to hot, or he does not sleep on top of it, then perhaps it can stay.
It is likely the Repti-glo is not putting out any UVB at this point, in part because it not as good a brand, and because they just wear out after about 6 months.
So focus on replacing the fluorescent tube light with a new, better quality bulb for UVB and a basking light on one end of your enclosure for heat, a warm side.
The other side of the enclosure should be cooler, around 20-22c.

Getting the UVB and heat right over the winter, along with a better diet, sounds like the first step.

Good for you to take this tortoise on!