I would just experiment. You're just looking for traction, so the ball rolling out isn't a concern. 500 or 800 would be my start point. Go big, if that don't work, try something else.

I knocked the polish off of my true blood with a 2000 grit pad. It definately skates way less than it did polished.

I see no way that ball surface can have any impact on bumper bowling.  What weight ball is he throwing.  Biggest impact on bumper bowling is the weight block of the super light balls.  If they are thrown hard enough they bounce off the bumpers. If thrown slower the weight block makes them hug the bumper rather than bouncing so much.  Weight blocks vary a bunch in those light weight balls.  It is the difference in top weight that is changing the reaction off the bumpers between the house ball and the ball you purchased

Also maybe your house is different, but I have never seen light weight urethane house balls on the rack.  They are almost always polyester. 

At 4 years old, you just want your kid to have fun playing a game. I'm 31 and only want to have fun. My son is 4, and has an absolute BLAST watching pins fall. Don't make it more than it is. Let your kid be a kid.

<Rant off>

I have a 6 year old great grandson.  He likes to go bowling, but isn't interested in league yet.  Maybe he never will.  I get him an 8 lb house ball.  If he throws it a little slow it will head toward the bumper and not bounce off hard enough to get more than a couple of pins.  If you watch the ball go down the lane you can easily see the effect of the weight block.  Every house ball will roll a little different.

Also, I am pretty sure the Eagle house balls are polyester, the same as the Maxim.   

I get him an 8 lb house ball.

8lb ball for a 4 year old?