I wonder if it's sensory issues 'kicking in'? If it is, then to him it would feel like this:
"Oh no, change of routine - we're going somewhere! I've just had a whole change of routine over Easter because people visit and do things in a different order and it's so scary, but I know I've got to behave so I'll cope somehow. But today, we're going to see friends and I'm already SO tired from all the routine-changes. And now my clothes hurt and it's hotter than the very desert in this car.
The noise from the engine, the smell of the petrol, someone talking to me - I can feel me getting more and more and more scared .
These jeans SO hurt me. I've got to get them off...I've got to find a way to cope. When we get there, there's going to be people doing the scary eye contact, and people asking me questions and jostling me and it all hurts, and I can't handle it. I want someone to make it all stop. Oh no, mum's talking to me to make me put the hurty hot jeans and pants on again. Can't do it, don't make me do it, it hurts, it's so hot, I don't want to go in the house when I'm so tired and scared.
Oh no, mum's shouting at me, it sounds like an explosion because my hearing is so sensitive when I'm like this. I want to hide, I need to hide. No way am I coming out of the car to face that hell..."
Same principle with scratchy pyjamas or any other change of clothing. For us it can be like taking off several sheets of sandpaper and putting on more sandpaper. Baths? I won't get in one. No way. Even as a adult. Echoey, cold, splashy, smelly-lotioned, huge contrast between temperature of water and the bath...it's a sensory nightmare. Others with an ASD may not agree, but for me it's a big thing. I'll do showers, but not baths.
That's how the world can look from our perspective.
Visual charts can help, because we need to plan how much energy we need to use, when. We can save a bit to cope with the visit to Aunt Esmerelda that way. A quiet corner for us to be in can help. Very low sensory stimuli can help - think "what perfumes are in here?" "are there flickering lights or screens?" "are there lots of children chattering and screaming?" etc.
In a meltdown, extreme quiet and something to wrap himself in to calm down. It might help? And a very large cuppa for you.