Long Question: It's about my ex and his new girl (who was his ex before me), should I trust them with my son?

As a mother who's separated from her baby's father, you probably will NEVER trust him or think he is a good enough parent. However, if he is willing to ist with this child you chose to have by him, you should ethically do so for the sake of your child. If this is or becomes court ordered, you are legally obligated to follow. Either way, your son can decide at some point whether he wants to continue.