Ok so, it sounds like he has reached his limit. Techniques for a neurotypical child won't work for a neurodiverse child in the most part. What has worked for us is reducing all demands except essentials, like brushing teeth a min of once a day, a bath every 4 or so days, and eating healthy-ish food. So no chores, no pressure to do activities, no pressure to engage with people, no need to wear clothes in the house. Just chilling out and following the childs interests. For one of my daughters this was painting and is now playing minecraft and watching youtubers play minecraft, for my other daughter its drawing, playing with horse toys or playibg minecraft, and for my son its endless kids youtube videos or going to buy a little toy for his collection- a train, dinosaur, or minecraft figurine (minecraft features heavily in my kids special interests!). The anxiety is often triggered by a build up of stresses rather than a specific anxiety inducing event. So there is no one thing to 'fix', just that their little brains and bodies have had enough and cannot cope anymore. I am also autistic and have similar times myself, and its always a warning sign that there is too much going on. After a period of resetting, things get easier, but for us and our household its been a whole mindset shift, away from "this is what we're meant to do as a family" to "this is what works for us as a family and means we have calm happy children".
Does your son have a comfort/ safe food that you could feed him everyday for lunch for example? So that he always has his familiar favourite and doesn't have the anxiety of not knowing whats coming up.
Avoiding "how do you feel" questions has made a huge difference here too. My kids behaviour shows me how they feel, and I put it into words for them- you seem frustrated, you seem tired, you seem worried- and I don't expect them to use their words if they are struggling, speaking is optional.
Drowsy piriton works here as a reset too. I would be wary of anything stronger like Phernegan until you know how he reacts to Piriton tbh- in neurodiverse people, drowsy medicines can have the opposite effect and cause hyperactivity and anxiety. Piriton works for 2 out of my 3 when they need a reset on sleep, but not for my other one.