I started my son out with a spring piston air rifle when he was 10. It was perfect for teaching him some basic gun safety, backyard plinking, and even squirrel hunting. He can't hunt with a firearm in Michigan until he's 12 and takes a hunter safety class. Well, he took the class this winter, and he turns 12 next month, so I found myself asking the same question as you are.
I couldn't decide between a 20 gauge and a .410. I didn't want the recoil of a 20 gauge to make it uncomfortable for him, but I also didn't want him to become discouraged because he missed every shot using a .410. I briefly considered the 28 gauge, but decided against that option because of the limited choices of ammo. The cost of ammo could be a factor for you as well, although if you're decision is between a .410 and a 28 gauge ammo cost will be about the same.
I bought my son a Pardner .410 because we'll be doing more squirrel and rabbit hunting. Then I bought a Pardner 20 gauge for myself. That way, he can try the .410 and the 20 gauge, then use the one that HE is most comfortable with. I was going to buy a 12 gauge barrel too, but for just a little more I bought the 12 gauge Pardner instead.
Every boy is different, and you know your son best, but if he's almost 9, a little guy, and shy of recoil, I'm confident that he will not like shooting a 20 gauge Pardner until he's a little older (I know you said 28 gauge, so I'll come back to that later). I took my son to the range this week with our three Pardners (.410, 20, and 12). He shot some squirrel shaped targets with the .410 using 2.5 inch #6 shot loads and he loved it. Next, he shot some 3 inch slugs through the .410 at paper plates on the 50 yard range and he loved that too. Then he tried a couple of target loads with the 20 gauge and didn't care for it. Next stop...the trap range. Since he had never shot trap before, he wanted to "watch the first time". After I shot a round with my 12 gauge Pardner he was ready to give it a try (we were the only ones there). I explained to him that he could use the 20 gauge and probably break more targets but the recoil would be worse, or he could use the .410 with less recoil but it would be more difficult to break the targets. He chose the .410...didn't break a target but had a ball...and blamed his lack of success entirely on the gun! He's looking forward to giving it another try.
From what I've read, most guys consider the .410 an "expert's gun" when used for moving targets and a "beginner's gun" when used for stationary targets. If you were chasing squirrels or rabbits, I'd say go with the .410. Since you're taking him on a dove hunt, I'd be tempted to say go with the 28 gauge. I've never shot a 28 gauge either, but I did find a Shotgun Recoil Table that had some interesting information there:
http://www.chuckhawks.com/shotgun_recoil_table.htm.410 bore, 2.5" = 7.1 ft. lbs. recoil energy
.410 bore, 3" = 10.5 ft. lbs. recoil energy
28 gauge, 2.75" = 12.8 ft. lbs. recoil energy
20 gauge, 2.75" = 16.1-25 ft. lbs. recoil energy (depending on load)
Based on that information, and my son's experience, I'd say that he would be OK with the 28 gauge. The recoil he experiences with a 28 gauge should be a little more than the .410, but also significantly less than a 20 gauge. The modified choke and increased payload of shot should increase his odds of success.
Good luck, and let us know what you get.
Moonlitin