1- Follow the instructions on the milk replacer to a tee. Too much is as bad as not enough.

2- calves butt their mothers bag to stimulate them to let down their milk. As they get older it's a good idea to stay outside the pen and feed like Jody said.

3- Don't put him in with the horse. I'd keep him separate until he is old/big enough to be weaned from milk

4- If he is eating hay and doing well, turning him out is ok. But if he can't compete with the bigger cattle he will quickly fall off. I'd wait til spring and he's at least 200-300 lbs before turning him out. That way you can make sure his nutritional needs are being met. Example- grain him with the herd and the cows will run him off and steal his feed.

Good luck to ya and welcome to ranchers.