Sleeping troubles

Hey guys

So our little one is almost two. At one we started a strict schedule with him for bedtime. Bed at 730, with us reading to him while he’s in the crib. He would fall asleep after we left the room. The past couple of weeks have been ridiculous. He will cry forever when we leave the room. He’s not overly tired or hungry or sick. We adjusted his bedtime to see if that would help; it hasn’t. He doesn’t like to be rocked to sleep. He usually cries for 15 minutes every night. Even if we let him back out of his room for a bit longer, he still cries when he goes back to bed. Any ideas or tips?