Kid landed hard but not sure what to tell him to correct it

Last weekend I was at the track with my kid riding. He rides a KX65 and has been on it for 6 months now and making some decent progression, he had only been riding for 2 months before getting the 65. So he's still really new.
Well Saturday he was really getting after it and having fun. There's a tabletop on the track that is a lot of fun and forgiving. I have to hit it 3rd gear mid throttle to clear it on my 250. He was 3rd gear WFO on his 65, I was following right behind. He launched it and came up maybe a foot short on the jump and landed hard, but clean. We finished the lap and he pulled off saying it hurt his neck, guess it whipped him a little. I chalked it up to landing hard and he was wearing a brand new helmet, thought it is Bell Moto 9 carbon Flex so it should be a bit lighter than the Fox V1 he has been wearing.

Anyway, he rests for a few then we go back out for some more laps. Same jump and he nails it again landing in the same sopt, same way. This time he pulled off the track immediately screaming that he hurt his back.
I was directly behind him and he landed fine for the impact, the bike didn't bounce funny or anything like that.
He didn't hold his legs though and bounced off the seat so it likely transfered some of that impact into his back.
By the time I got loaded up and we were leaving, he was feeling better but said he could still feel it.

We were supposed to ride Wednesday night and he didn't want to, which is extremely odd for him. He told me it scared him a bit. So I'm trying to figure out what to tell him to convince him he's ok and to keep pushing. Also trying to explain to him how to hold his weight up with his legs. He's a big kid for his age and he's growing at a faster rate than he is building strength. I told him we are going to have to build a workout plan for him to get his strength up, but nothing crazy as he's only 9. He has to be strong enough to hold on to the bike and he's not for the speed he's going now.

In an effort to explain to him how to land off of jumps and use his legs, I have him squeezing a mini basketball between his legs and jumping off of stairs. That will mimic squeezing the bike and holding his weight when he lands.

Other than that, I'm not sure what else to tell him. Am I doing the right thing? I know there's some moto dads here that's worked with their kids. While I can see what he's doing right and wrong, it's not always easy to convey that to a 9 year old that just loves riding bikes and playing in dirt.

He's also doing a MX camp in a couple of weeks, so I'm sure he will pick up some good things from some of those guys.

Thanks