Your friend reminds me of Job's so-called "friends" in the Bible! You may remember that Job was suffering very greatly (both physically and mentally), and these men "comforted" him by telling him God must be punishing him for some terrible sin. But they were wrong.
Why did God bring this child into your lives? I have pondered questions like this repeatedly over the years, and to be honest I don't have a complete answer. Someday we will understand—but not now. The Bible says, "Now I know in part; then I shall know fully, even as I am fully known" (1 Corinthians 13:12). We live in a fallen, sin-damaged world, and life isn't the way God meant it to be.
But I do know this: God has given you a special opportunity by entrusting this little boy to you—an opportunity to be a channel of God's love to him. Years ago, some friends of our faced a similar situation, and ended up seeing their child as an "angel" sent from God to bless their lives. May this be true of you.
I know your path isn't easy—and God knows it also. But He loves you and wants to help you become the best parents you can possibly be to your son. Make Christ the center of your home, and then ask Him to help you convey His love and protection to this child who needs you so much.