My name is ***** ***** I am an experienced criminal
All but 5 states belong to something called the Interestate Driver's Compact, which is an agreement among the states to hold drivers accountable under a one driver/one license theory.
Hawaii cannot suspend your son's license. All they can do is take away his driving privileges. However, under the terms of the Compact, California has the ability to treat the Hawaii offense as if it is its own, and they can impose additional penalties on your son, based on their assessment of the situation.
In other words, after your son finishes all of his Hawaii penalties and waits out any Hawaii imposed suspension period, he will need to show California he owes nothing further to Hawaii and then jump through the hoops California requires for reinstatement.
I know this sounds unfair and that he's being punished twice, but I can assure you that this is how the law works with DUIs. Exactly what California will want him to do is at the discretion of the California DMV. They will likely want him to complete the CA safe driving classes. They could require that he install an interlock. They could add an additional suspension period.
This is going to be a complicated process, and the requirements are not the same for everyone. Much depends on the facts and circumstances of the out of state case and his sentence
for that crime. He will need to call the Hawaii DMV and find out what official proof Hawaii will require that your son has his driving privileges again. Then he will need to call the CA DMV, ask what they will require of him to reinstate his CA driving privileges and get it done. He should make it clear that now he lives in Hawaii.