Can you put him in another field? If they can't be separated, they still sell those clip on nose weaners. Not nice, but the weaner plate is clipped into the calf nostrils and covers the calf mouth so he can't get a teat when his head is tipped upward.
Here is the modern version, looks easy to apply and pretty cheap!
QuietWean Calf Weaner | PBS Animal Health
You may want a couple on hand, in case they get broken or you have several calves to do in the future. Just one shipping price then.
Interesting trying to find a photo of what I remember, and learning the "new" method and reasons behind it. Seems that calves wean easier (less stress) and quieter, using these devices and left in with the mothers for a week or two before separating them to sell off. This time wearing the weaner devices while still with the mother cow is called "pre-conditioning". Cows still have their calves, but dry up with no nursing. Calves get to eating well on grass, hay, grain with no milk, can hang with mom, but tend to group with other calves. So using the weaners a big help in reducing scours, animals going thru fences, related problems of stressed calves when they are finally parted from the mother cows.
And one person said such devices were always called "blabs" in the past, for the bawling of the cattle at weaning time. All the reading behind using the device was quite interesting. I learn something new all the time!!