If he is a stray, he's lost his mommy and his brothers and sisters. I'd meow and cry too.
He's objecting to being left alone. He's afraid you won't come back. All I can suggest is that you talk to him in a low calm voice, and tell him that he is safe now, and that he will have food to eat and a place to stay until a home is found for him. He won't understand the words of course, but hopefully the low, calm tone in your voice will make him feel better and he might stop crying. Don't forget to hug him and tell him he is beautiful and loved.
By the way, bless you for taking in this little one.