25 year old disabled man gets a letter from Social Security saying he was overpaid $747.60

My 25 year old disabled son, Matthew got a letter from social security saying he has been overpaid $747.60. The letter says the income on their records was wrong and that Social Security uses income from 2 months earlier to calculate his SSI payment. For example, they use income from January to figure the payment for March. As Matt’s guardian, I submit Matt’s income from his job at Portillo’s as a lobby cleaner every month on the social security app and I get a receipt mailed to me each month confirming my submission. I also got the same letter as Matt’s guardian, but my letter said if he doesn’t pay the amount back, as his guardian I would have to pay it back for him. This is not the first time Matt has gotten a letter like this. The last time, Social Security deducted the over payment from his monthly checks for six months.