that's what Im told maybe his heart wasn't using it

I had pacemaker installed and they said 7 to 10 years. It has been 3 and I need to get it replaced because battery is low.

Pacemakers are usually designed to do several things. For the most part, the pacemaker should not need replaced ever (except the battery which will need to be changed about every 10 years). The doctor can interrogate the pacemaker and make a decision, but 20 years seems like a long time.