Is he with the cow all day? Separate them for the day and then get him along side to suck, get him right in there and squirt it from the teat to his mouth. Do it a few times, may just have to squirt it on his mouth and nose at first and stick your finger in and rub the top of his mouth, don't pry its mouth open as it just wants to back off then. Squirt it in while you've got your finger in if you can, until you can swap finger for teat
If it won't catch on being hungry and squirted right in its mouth to start on its own - its too dumb to live. I've had them need help for the better part of a week, but it was because they were too weak, or their legs were too messed up, but they caught onto the sucking part after a few days if you could hold them in place. I don't have time to deal with it beyond that and they're on their own.
They seem to become lazy if they know food will show up and get poured in their mouth, rather than have to stand there and suck for it. That and they may just be losing the instinct to figure out sucking, when its provided.
I use an old lawn chair, or upside down bucket to set on while I work em, as long as the cow is quiet. You get frustrated faster trying to hold yourself and a dumb calf up, but if you can sit and work at it, it goes easier. But I did also get kicked in the teeth last week )lightly) dealing with a dumb sucker too. I was only kneeling next to her though.