My 4yo recently had grommets- we let her choose the method, and I'm glad she went for IV as I think she would have been seriously freaked out by the gas mask (she has serious issues with going to sleep!)
She had 'magic cream' put on the back of each hand to numb them before insertion. We were offered a pre-med but as a back-up IYSWIM, and she didn't need it.
Putting the canula in was still painful for her, but she was very calm, and allowed them to do it, and they hid her hand from her by standing begween the bed and her hand - and distracted her with toys and questions (though she wasn't all that distracted, but fine because we were there)
The 'magic milk' (anaesthetic) worked so quickly when they injected it, that she didn't realise she'd been to sleep afterwards bless her!
Ours is a children's hospital, and they speak to the patient and involve them in their treatment and decisions all along- it really helps, and DH and I being calm and supportive helped too (though I was terrified inside!)
I am not sure how your DS's communication issues manifest, and how he is in temperament, so obviously can only share our experience. I hope all goes well- the difference having grommets has made to our DD's life is tremendous... I am so glad we did this.