Ssi benefits for level 3 5y/o non verbal son

My son was approved for SSI about 2 years ago, but after the approval letter we never heard anything else. The letter said he was approved but didn’t include a benefit amount or payment information.
Since then, I’ve uploaded all requested supporting documents and even opened the checking account for direct deposit, but nothing has happened. No payments, no follow-up letters, nothing.
Has anyone else experienced this? Is there a way to find out what happened to his case or if it somehow got stuck? Any advice on what steps I should take would be greatly appreciated.