I'm confused. Isn't a booster jab a third vaccination anyway?
The current situation is that, for the vast majority of people, a booster jab will either be a regular dose of the Pfizer vaccine or a half dose of the Moderna one. Anyone who had the AstraZeneca vaccine for their first two jabs will normally be switched to one of the two alternatives for their booster but, where there are contraindications to the use of either of those jabs, special arrangements can be made for AZ to be used again if needed.
I hope that your son will feel better soon.