Kids dissolving into tears when they are being picked up is a form of release and is a sign of separation anxiety. They are so overwhelmed to finally being seeing mom again that they can not contain themselves and the emotions overflow. Opening the gate, the doorbell ringing, moving to one area over another are all things he associates with mom coming. If you go over the gate you are gone as in he is separated from you by the gate. The good news is that the phase tends to pass fairly quickly. Treat it matter of factly, talk to him from the other side to let him know you are still there, ignore his protests and just assume he has the inner capacity to cope - he is totally lacking in confidence. Unlike most recommendations I would not make any comments to him, special contacts etc when he is coping and not needing you. All that does is remind him that he might have options if he protests. Has this been going on as long as he has been with you or is this recent. Ask the mom if they have recently put up barriers at home such as blocking off access to the kitchen or a gate at his room door in case he gets out of bed. He just needs to learn that he is ok and is a separate entity from his mom or other adult. keep your head up, avoid unnecessary eye to eye contact with the child and just treat him like he isn't putting up a fuss but give as much extra attention without it looking like you are if that makes sense. He needs to know that at least you aren't going to go away and leave him like him mom did.