It will definetly help with hills, mud, tree crossings, ect. He will get into the power much quicker (the change you are making is roughly 10%). What we discovered in gearing the bike down that much with a close ratio transmission, it seems to run out of gear in a hurry. In other words, the power band seems narrower, even though it's not. It also causes the throttle to be more sensitive to rider input. Sometimes thats good, others it isn't!
I was assuming you guys did MX. Knowing that it's harescrambles, I would more strongly recommend the port job. He will lose some top-end, but my experience has been there are very few kids riding 80's that can use all the power they make anyway. I don't think anyone will be able to run off and leave him in the environment he's racing in. Besides, if you don't like it, Eric will redo it. Since the YZ and CR don't have power valves, there aren't many choices to increase low end power. I recently saw a dyno chart that compared all of the minis and was amazed at the difference in the shape of the curves. The KX and RM make about 12 HP @ 8000 rpm. The YZ makes around 10 and the CR only 6. They all peak in the 17.5 to 18.5 range.
I just noticed you (y'all) are from Alabama. We are in the Birmingham area.