We have 6 children and the oldest 5 all took piano lessons, so I totally relate to practice battles! First, you have to be sure you care enough that he takes the lessons to be worth the battle. Then try making it fun for him - you sit down and say, OK, I know the left hand is harder so I'll do the left hand this time, then you have a turn. Or make him the "expert" - you know, Johnny, I was trying the left hand part and I keep missing this note - can you teach me how to do this?
Another option is to give him a choice - do you want to practice first or clean your room first? (Of course, it has to be a choice between two equally undesirable [to him] options!) You could try a reward chart - if you practice without complaining you get a star and 5 stars means we'll go to the park for an hour, or we'll make cookies together on the weekend. I try to make rewards special times with mom or dad rather than TV time or food rewards...you want relationships to grow! (baking cookies TOGETHER doesn't count as a food reward, IMO!) Once you're sure he understands what's required to get a star, don't nag - set the timer, let him practice, then if he was complaining, just say, "OK, your practice time is up. I'm sorry I can't give you a star today, but you can do better tomorrow." If he cries about not getting a star, remind him that HE chose how to do his practice, so if he wants a star, he'll need to choose differently next time. If he wants to try again the same day, I'd let him, but only one re-try the same day, and never nag or remind him of the stars because YOU want him to get one. (That's a hard one for moms!)
As far as the age thing, ours all started in Grade 2 except the second daughter, 4th child, who started in Kindergarten because she wanted to so badly. We had fewer practice battles with her than any - in fact, her sister, 8 years older complained because the younger one was playing "her" music! We just said, then I guess you'll have to practice harder to stay ahead of her! :) And yes, our boys did have to work harder on the dexterity, but they did much better on the days their attitude was good!
Anyway, that's my best advice after seeing 5 kids through - the good news is that all 5 still really enjoy music, 2 still love the piano, 3 took up other instruments after (or during) their mandatory 4 years of piano, and one of those 3 is coming back to the piano now. Child #6 has a learning disability so at 8 doesn't have the motor skills development necessary for piano lessons, but we're hoping it will come with time.