According to this article Ineos won't let him
the young British superstar has requested approval to race with a Red Bull helmet on the road, but the Ineos Grenadiers have repeatedly declined that request.
https://velo.outsideonline.com/road/road-racing/why-is-wout-van-aert-allowed-to-race-in-a-red-bull-helmet-on-the-road/
Ineos probably don't want their brand to be damaged by associating it with a caffeinated sugary drink.
@andyp started
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According to this article Ineos won't let him
https://velo.outsideonline.com/road/road-racing/why-is-wout-van-aert-allowed-to-race-in-a-red-bull-helmet-on-the-road/