Advice re head banging

Hi everyone, I’ve joined as we are looking for help/advice from anyone who has had an autistic toddler who bangs his head in when frustrated. Our son, who is 3 next week, started this a couple of months ago & it has become progressively worse. He now bangs his head at least 20/30 times every day & has bruises all over his head :-(. Have spoken to gp & plan to call his paediatrician tomorrow but thought parents who have dealt with this before might be the best place for advice on management. At the moment we are at the stage of avoiding saying no to him or upsetting him in any way just to avoid him banging his head, but it’s very hard since he offer doesnt want to do so many things eg having a bath/eating/nappy changes. Any suggestions from parents who have been through this gratefully received