Because of your little guy's age I would guess that he is sick/trying to fight something off and he's too tired/weak to eat or drink, which is causing the sudden weight loss/dehydration. You're already going to take him to the vet, in the mean time keep him warm and hydrated. He's likely too weak to hold his blocks, which is why he wasn't eating but you can make a mush for him (using water, soup, juice - not citrus!, Ensure, etc.) if you're having trouble finding foods he will eat. Don't forget that foods like yogurt (which he seems to still get exited about) also contain water and they'll help rehydrate.
My only other piece of advice is to see if you can find a vet open Saturday and get an appointment tomorrow (er, make that today). Tell them it's an emergency (it is, rats can go downhill quite quickly) and that it can't wait until Monday. Don't be afraid to be firm/pushy, I've made that mistake in the past and it didn't have a happy ending.
Good luck! I hope Jack feels better soon. :hugs: