What should we do about regression in autistic son?

Professional help. Your son needs a complete evaluation and comprehensive interventiuon, if he has already had that , stress, infection, if he is on cfgf diet, dietary transgression can cause regression. Get his team to put their heads together to identify what is stressing him. If he has not "team" he needs one.
Depends . The current age of your son, the age of diagnosis and any precipitating events are crucial to the decision making process. Your meaning of regression is also important. Do you mean loss of skills like language or the sudden development of anxiety, tic, ocd or agressive or sterotypical behaviors. Infections can be a trigger or environmental exposure. Speak with knowledgeable provider.