"The boy is drinking milk."
Translation:Le garçon boit du lait.
The form "be +ing" doesn't exist in french. You just have to remove the verb "être", and conjuge the other one. You can also see sometimes "be" (conjugated) + "en train de" + "verb" (infinitive), if someone comments an action being unwound: "Le garçon est en train de boire du lait", "il est en train de courir", "tu es en train de faire un exercice". But "en train de" adds a lot of words to the sentence, so don't use it to much. Just use the present, it will be correct most of the time.