It is there, even at iso100.
Things to do:
1. Try to expose correctly. This image isn't.
2. Shoot raw, shoot 18mp. Around 12 of these are redundant for most uses, resizing a cleaned image can hide a multitude of sins.
3. At the raw conversion stage play with wb exposure contrast so it looks like you want.
4. After previous tweaks reduce luma noise as much as the image can bear, view at 100% try and keep some fine detail in your loupe. In ACR I find 40-50 cleans up most images without affecting sharpness.
5. If you shot at high iso's the some soft chroma nr may also help.
6. Save as 16bit tiff take into PS open in Lab or channel mode and try slight guassian blur on the offending noisy colour. Always save the original and wirk on a copy.
7. Finally. Auto iso? C'mon.
The noise issue is common to the four 18mp cameras i've owned, though the m is a stop cleaner over the digic 4's.
Some folk get a bad camera. Most folk don't set the 7d up properly, for this kind if shot I would use zone mode, and probably the centre zone. Ai servo (obviously) and H burst.
I would shoot very short and sporadic bursts of h. Those gaps give the camera a much better chance if tracking and therfore predicting during mirror up.
Shooting 10s of h burst might work, but shooting track track burst, track track burst will give you less overal shots to trawl through, but a far higher keeper rate.
You can also experiment with the 7d's tracking behaviour, so that foreground objects are ignored almost entirely (oh yeah and set your lens limiter if you have one. You can also influence how long the camera 'predicts' for, should the mirror or an object obscure the subject.
The 7d is a great tool, but it demands of it's user. If you work together the results can be superlative, left to it's own devices it can be a bit wayward. So whats the point of an automated camera that requires so much forethought to use? Well it assumes a certain level of technical understanding so that the automation can be targeted smartly, it can make a good photographer even better but will embarrass anybody who wants the af to work well out of the box or use images straight off the card.
Master the 7d and your son will find it an adroit and willing companion for events exactly like airshows.