If you can get any actual electrolyte solution - Pedialyte, even Gatorade - get that down him.
For a more complete electrolyte solution, add a spoonful of salt and a splash of apple cider vinegar to the above, and cut the yogurt, although that doesn't seem to taste as good and they're a bit less likely to take it if they're already refusing a bottle.
The most important thing right now is to get him hydrated and stopping the scours, so you need to get and keep him drinking and keep his blood sugar up, but let his poor upset tummy settle, so no milk for a day.
So after 24 to 36 hours of straight electrolyte (until the stinky yellow scour poop has changed) then add some yogurt to it for a feeding, then do half electrolyte+yogurt and half milk for 2 - 3 feedings, then go back to milk, small feedings, frequently.
And go online and order the Resorb
https://www.amazon.com/Pfizer-BC040956-Resorb-Packets-Calves/dp/B000TZ8SSC?th=1&psc=1
It has saved more than on rescue baby for me, when homemade stuff was not cutting it.