Is There Any Way To Address/Minimise A New Trigger In An Autistic Child?

Hi. I'm new to the forum and straight off the bat I'm looking for any advice on a situation with a child I transport to and from school.

I'm a school run driver for autistic children. Our vehicle is snug - not a bus - so it's a fairly close environment for the five boys we have. They all seemed to gel, and myself and my escort are both pretty

involved with them during the run - singing, reciting the alphabet, doing magic on the traffic lights - baby shark's a massive hit with all of the boys. So it's a pleasant little run and they seem mostly comfortable with each other.

The issue we have is that one of the boys is just a bundle of energy - he has a diagnosis of autism but he does connect with us and the other kids, enjoys lots of interaction and can be quite hyper active. He's a friendly little boy of eight and gets excited when we spot trains on the way to school and shouts and gets a bit loud. Another little boy of five has been triggered by this, and though he likes the first boy the shouting became a real trigger for him and he started to get really distressed even in anticipation of this.

We spoke to the older boy, telling him that the younger boy likes him, but doesn't like loud noises and we would all try to be a lot quieter and use our talking voices etc. There were other behaviours like pulling on a seatbelt and touching the younger boys head (not aggressively - he was actually trying to calm him down, saying "It's ok - don't cry."). He responded amazingly to this and stopped these behaviours. He's also responded to us all being quieter and tries really hard to speak rather than shout.

He's been a little star and done everything we've asked of him but unfortunately it seems even the sound of his voice is triggering the younger boy now. He is using his speaking voice but it's setting off the younger boy even when just speaking. We obviously can't ask any more of him he's done so well already and we're at the stage where we know we have to try to encourage the younger boy to stop associating the sound of the first boys voice with noise.

Does anyone have any experience of something similar that would help to minimise his reaction to this other child's voice?  Any advice on this would be so appreciated.