Call the vet. It sounds like he could possibly have coccidiosis which is slow eating and sometimes they go down. What about his manure? Bloody, runny, scoury? In a calf that young it could be different things and unless you see him in person it is very hard to diagnose. Also, once in a blue moon a calf on milk will get bloated, possibly from a "bloom" of bad bacteria or something and could cause a "bellyache" and cause them to go down, like colic in a horse.
Don't know where you are but in the cold that has been gripping the nation, it could kill him quick. If it is something like coccidiosis, dehydration and the cold will kill him quick.