It is definitely possible for him to adjust but I am curious as to what the issues you are having are?
If mom thinks it's all normal, then I would absolutely tell her that if she does not want to send him additional days then you'll have no other option but to call for pick up every time the crying/wanting to be held goes on for more than X number of minutes. My limit is 90 minutes.
If a child does not calm down and join the rest of the activities within a 90 minute period, I refuse to be the one that has to "deal" with it. It's unfair to you, the other daycare kids and to the child himself.
Once things become an issue for someone other than you, the parent usually always changes their behavior. Up until now, this isn't an issue for anyone other than you.
