Montessori does not always mean the child's emotional needs will be effectively met though....montessori is just about the learning approach.
And there is no such thing as being 'overly attached'......these are the characteristics of a secure attachment and are all things that the OP's child is demonstrating.
Proximity maintenance: The desire to be near the people we are attached to.
Safe haven
: Returning to the attachment figure for comfort and safety in the face of a fear or threat.
Secure base
: The attachment figure acts as a base of security from which the child can explore the surrounding environment.
Separation distress
: Anxiety that occurs in the absence of the attachment figure.
Alot of people don't understand that an insecure attachment is more likely when children spend too much time away from their parents not too much time with them!
OP you may find this video useful by Sarah Ockwell smith;
https://m.youtube.com/watch?v=6i1Uv3ZT-0k
I think doing the Berlin adjustment model she mentions to the letter would be rather difficult to implement and not many settings would be on board with that, however with my dd2 who went into childcare with a childminder at 10mths, we did do a gradual process of me leaving her for lengthening amounts of time over a period of time, which worked well.
I would say though (as both a parent and someone who has worked in childcare and also has professional experience and extensive knowledge about child development and attachment) that if your child is not settling down quickly after you leave and is so unsettled they are calling you to collect him, it is likely not the right setting for him.
Boys are often harder to settle than girls, but no child should be crying for any period of time and being left inconsolable.
I have left my DD's slightly upset with a caregiver they are familiar with and have built up a relationship with, but wouldn't leave them upset with someone they barely know (which sadly alot of settings expect you to do) or expect to be asked to leave them if they were full on screaming, especially as a baby.
In a situation where they were upset at drop off I would at least wait outside the door to ensure they settled and i could no longer hear them crying before I left.
In your situation, I would either have a conversation with the setting about going back to square one, with you staying for afew sessions and gradually pulling back, and trialling leaving him abit at a time (bear in mind this could take weeks or longer though as he has already had a bad experience) or of they won't agree or it doesn't work then take him out, and if you need the childcare for work purposes, perhaps find a good childminder where he will receive a more nurturing approach in a less overwhelming environment.Just ensure before agreeing to send him that they are on board with a gradual settling in process and won't be trying to force things along too fast, as I think that will key to avoid a repetition of your current experience.
Best of luck, it must be really hard for you and you really sound like you're trying your best for him.