I had one just like this. She would just stand at the door and cry and scream for mom. The odd time I could get her to sit on my lap and cuddle. For 3 weeks she sat at the door and cried with very few breaks. She would always come for lunch and she would nap fairly well, but as soon as nap was done she was right back at the door. And mom did everything right! No lingering at the door, quick kiss and hug, ect.

She eventually warmed up and the crying started to get less and less. I've had her for over a year and she's one of my favorites.

I think you need to let him work through it. Offer him hugs by the door if that's where he's comfortable. Remind him mommy will be back after nap and continue to remind him that he can come and play whenever he's ready.