Oh, OP, hugs to you.
It sounds like you have been doing an amazing job so far and it must feel so confusing and sad to have a big 180 flip happen like this.
As PPs have been saying, it could be a hormonal thing, or could be that something's had him look at his life story through a different set of lenses (might be the spat at the picnic, might be something totally innocuous).
Either way, he's riding a bit of an emotional rollercoaster right now and while he's on it, the best thing you can do is be a super-boring, reliable, predictable, loving anchor.
Don't ramp up anything with grand gestures to try and 'win' him back, don't disappear, don't feed the drama, just be the same-old loving dad and stepmum he's always had. Lots of space and opportunity to talk, lots of room to be angry until he works through it, and lots of boring old routine and love. Keep showing up for contact, if he doesn't want to come out that's fine, but Dad's outside in the car ready to pick him up either way. Plans to go to sports at the weekend are maintained, etc etc. All totally business as usual, and buckets of love whenever he shows up to receive it.
His mum, unfortunately, doesn't sound like she's doing anyone any favours, but your DSS will see that in time too.