I'd say stick it out till Wednesday and if he's still feeling bad then you'll collect him. That way he feels it doesn't have to be so long, and then will be fine.
I've just had similar from my 8yo ds who's just gone to his first camp on his own. Both dd1 and dd2 have been from his age, and love it-they count the days down from finishing one year's to starting the next!
He's much more a home bird, and hasn't stayed more than one night away before, so he was varying between saying he was excited and not wanting to go.
He was okay until it was time to get in the car, when his lip started going and he clung to me. (dh was taking as ds is much more inclined to cling to me so we thought it would be better)
They left and dd1 burst into tears and I wondered whether it was a good idea or I should have left it another year.
Dh said he was smiling 10 minutes down the road, and had to be interrupted at the camp to say goodbye.
I wouldn't be surprised to hear there were tears at bedtime tonight, but I think by the end of the week he'll be absolutely fine and loving it all, and desperate to go back. And I think he'll have gained a lot by doing it.
Otoh the girls have both been saying they miss him and wish he was back. 