We have raised over 20 orphans. Some were a day old and others were 2 to 10 weeks old. If they are 5 or 6 weeks old, we put them on milk replacer in a bucket. We have always used Land O' Lakes calf Milk replacer that is 'all milk' and no Soy. It should be 'white' and not creme or yellow in color. We feed it 'cool' and feed one that age 3 times a day and late evening before I go to bed. We have used the milk pellets, but have had much better growth and condition when we used milk replacer. All of our orphans have grown as good as the other colts that had mothers. They were not pot bellied or ribby.
We handle them very little. We halter break them and leave them alone. The last thing we want is a colt that thinks they should identify with people. As soon as they hit 2 1/2 to 3 months, I take another colt of ours and wean it and put them in together. If I don't have another colt the same age, we go to the sale and buy a colt or a mare and colt. After a 2 week quarantine away from the orphan, we wean that colt and take the mare back to the sale and raise the two together.
I might mention one other thing. So many orphans get so pot bellied. They have to have a very healthy population of 'good' bacteria in their gut. They get it naturally from eating poop -- usually their mother's. So, when I have an orphan, I see that they get a fresh pile of juicy poop in their pen every day. I get it from a big, slick fat 'easy-keeping' gelding that I know has no parasites. This is natural and normal. It makes one have a better appetite, gets them eating solid food more quickly and keeps them from getting digestive problems.
Hope this helps.