What are the things I should be aware of?
That he'll probably expire from something else before his pacemaker battery need replacing!
Depending on the duty cycle, a pacemaker battery can last from 5 to 10 years.
I misread your post. He has 2-3 months REMAINING on the battery. The device fires an electrical pulse to either make the heart or assist the heart to contract, just as the heart's natural electrical system does. Depending on how "defective" that electrical system is, it's possible the device could stop working, yet his heart could continue to function. I think there's a lot of unknowns in providing you with a more accurate answer.
...I keep thinking of additional comments.
Modern pacemakers don't just "die". When the battery voltage drops to a certain level. The pacemaker will go into an "end of life mode" (battery, not person). It's sort of a last ditch conservation mode until the battery can be replaced. What happens then is the pacemaker will fire to a preset pulse rate. It will no longer take into account what the user's heart is doing. The is reported by some to be a very uncomfortable, miserable feeling.
What percent of time is he paced?