Well, I never actually went on disability. I lost a couple of days when I was treated and 3 days (+ a weekend) when I had a severe reaction, but not much else. How much time you lose depends on the treatment and mine had few side effects (except for the unusual severe reaction).
I think your boss sees this as extending forever. If it's been 2 months since treatment stopped and you are still fatigued I would suggest talking to your doctor about treating the fatigue so you can go to work. Depending upon your "read" of your employer, I would suggest talking to him. Say you want to come back but be honest about yout fatigue concerns. Try to suggest reasonable work-arounds if they become necessary. This may depend upon what it is you do. A police officer or a sales clerk (for example) probably can't afford to be less than 100%. Many an office worker can. Not every boss will be happy about an employee who isn't 100% but you might gain "points" by making an effort. Also, it may be that making the effort is the fastest way to a more complete recovery.