You are incorrect. The only plea your son should make at his arraignment is a plea of not guilty.
He has that right under the Constitution, as he is presumed innocent until proven guilty beyond a reasonable doubt.
Pleading not guilty is the only plea that keeps a defendant's rights open long enough for him to receive legal counsel and understand his rights and choices well enough to make an intelligent decision about his case. A plea of guilty or a plea of no contest will give your son a criminal
conviction to put on his record, and there would be no reason for him to put himself in that situation until he knows what he will get as a sentence
in exchange for the plea. More simply put, if he pleads no contest and asks for diversion and the DA says no, he's stuck. He's already admitted guilt on the record and the negotiations are out of his control.
I don't know how old your son is, or the value of what he's allegedly shoplifted, but in any case, there is nothing much worse than a theft offense, even on a misdemeanor
level, to ruin someone's future. The offense goes directly to dishonesty and lack of character, and employers will simply run away from him. For that reason, he should have a lawyer, who will be in the best position to protect his rights, since criminal cases have lifetime consequences.
If he can afford one, he should have the lawyer with him on his first court date. If he's unable to pay for one, he can plead not guilty and then ask the court to appoint him a public defender.