Never been more sure of an armchair diagnosis than this: agree it must be crohns.
I wouldn't wait until Monday, I'd try and get him admitted earlier for stabilisation. No they won't start treatment but they can give fluids, pain relief, prescribe meal replacement drinks etc
Perhaps even get him a ct scan.
I wouldn't take him tonight or tomorrow evening, you'll be hours, but Saturday morning or early afternoon you should be OK.
It might be better to attempt an out of hours urgent apt with a GP first, who can then refer him directly to A&E which will also speed things up once you arrive.
Of course, if you get a gp who says to wait it out then you can still go to A&E anyway.
Do prep for A&E - take an overnight bag for him (can always keep in car until he's on a ward), take water/hot drinks, snacks, phone chargers - and anything which will help his ASD too like ear plugs and headphones.
If you decide you're ok to wait until Monday, then have you tried nutritional drinks? You can buy from boots - Complan, Ensure etc. They don't taste great but some people like them more than others so it's worth a go. And some people like some flavours but not others. Talk to the on duty pharmacist for advice first though.
It's important to keep his fluids up - rehydration salts for diarrhoea will make a huge difference to how he feels. Some taste better than others - personally I think boots or superdrug own taste best & drinking from a bottle rather than a glass can help because it stops you smelling them.
Sips of water may be more manageable and if he can eat at least one thing a day that will help too. Plain egg sandwich on white bread was my brother's go to (severe ulcerative colitis at the same age, a weak barely-walking skeleton like your son). Just a boiled egg in soft white bread. Gentle, smooth soups were also just manageable.
Even when hardly able to eat, any sustenance is better than none and does make a difference to strength etc.
Good luck and if you get scared about waiting or find yourself dithering as to what to do - just take him in. Even tonight if you need to. You won't be wasting anyone's time.