Try setting up visual cue cards with expressions of emotions like happy, sad, mad etc. Each emotion with different colour. Blue for sad yellow for happy red for mad. Make up cue cards for food, chores and use them for him to communicate with you when he is all upset and in the middle of his tantrum. Autism is difficult for everyone involved including the child. Making things easier for the child lessens meltdowns. The diaper will come off at his pace and at his self confidence level. Knowing when to go to pee-pee or poo-poo is a hard mental task & takes a lot of work with praise. Keep it up as you defitinely have a calming presence if the boy prefers you to watch him.