If your ex-husband is a normal man, i.e. does not drink, adequate and wants to see your child after your divorce, I think, we should not prevent this. With him you need to stipulate all the conditions of his communication with your common child. Try not to go over to individuals and do not offend each other, and build barriers that can hinder their communication.
If the biological father will be bad about the appearance of another man's life in your with the baby, then treat this adequately. After all, you often observe how ex-husbands often steal their own children from their mother and this ends tragically. To avoid such cases, never forbid your father to see his child.
If your child wants to call his father his own, then try not to interfere with this.
And to his stepfather, he can either call by name or also call father if the child agrees that he will have two fathers. Also, let your ex-husband participate in the life of your child, helps you with some worries. For example, walking with him, leads to various sections and stuff. When your partner or your real husband bought a gift for a child, tell your ex-boyfriend that when he finds out from another person he does not get angry.
How to properly introduce the child to your future husband? If the baby is still small, about 5 years old, then acquaint them gradually, do not rush. Meetings are organized not at home, but in some other place, say in the park, when you are walking or in a cafe. When a foreign man appears in the area where the baby is, it will be a shock for the child and he will not be able to get close to him. A man you must show that your child's opinion is important to you and you can not give it up. If your kid notices that you are not paying much attention to him, but paying all attention to "that uncle", then he will begin to be capricious, stimulate illness and so on.
When your child is already accustomed to your waking companion, then you can ask him if he does not mind if this "uncle" comes to visit you. If the meeting took place, leave them for a few minutes alone, let them get used to each other, talk. You can also send your child with him somewhere, say, to the store for a loaf. So they can get closer. If your baby is a little shy himself, do not worry, he needs time to get used to another man.
If the child does not want to call him "daddy", then do not force. Let him call by name or uncle. Always take care of your baby, do not forget about him.