My neighbor's son is ADD. When he's having fun and it's time to go soon, he gives him a 30 minute warning, a 15 minute warning, and a 5 minute warning. He says that works well. In your case, it might be a control issue. Since he's developmentally delayed, you might be in an extended version of the terrible twos. If that's the case, it's important to let him have his way sometimes, but never because he has a meltdown.
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"The cordial quality of pear or plum
Rises as gladly in the single tree
As in the whole orchards resonant with bees."
- Emerson