i had one like this... she started at 12 months and would stand at the front door and cry all day. My only saving grace was that she ate well, but would still cry as she was eating. She did this behaviour for 3 weeks! Yup... 3 weeks of walking to the front door crying.
I made the mistake of taking them outside before pick up one day so mom came through the gate... guess where she stood when we went outside after that day? yup.. right at the gate.
One days she just stopped. We went about our daily routine and if she wanted to stand by the door and cry that was up to her. She had the right to express herself... just like I had the right to ignore it. LOL
She's great now... sleeps 3 hours at a time... eats like a champ... comes to me in the am and says bye to mom... she even walks around her house all weekend saying my name. LOL
Hang in there... it's so hard to see them upset, but it's a process they have to go through. And if mom decides to pick up mid day then all the power to her... less noise for you to deal with in the afternoon. Don't try to force anything... he'll get comfortable in his own time.
Oh! And have a bottle of wine always ready to pop open at 5:30 when all the kids leave... it helps.