Oh OP, you write so beautifully and as a mother of a 2 year old boy, I find myself welling up just reading the story. I send you so much warmth and support as you begin to heal from what happened. You deserve to heal, and all I can say (easy for me to say as an insider) is that I so strongly believe that your gorgeous little boy will be so unaffected by this attack. He is tiny, his brain is so flexible, and I really believe the ongoing pain is all yours. Horrible for you, but comforting in some ways.
Your little four year old - I rather feel it is wonderful that he can talk openly and regularly with you about what happened. Surely that is how he processes and dismisses it all.
Onto your question about what can be done...I am in London, and I do not know if you have the finances or insurance to cover any of this.
But a few enormous recommendations from me:
David Floyd is part of the plastics team at the Portland Hospital - specialising in both children and women. When my step daughter nearly ripped her ear off in a sailing accident (could have had life long physical and cosmetic impact, the ear was half dangling), Floyd operated. He did the most beautiful job and no one could have said more about his care, attention and focus on getting her the best result she possibly could.
www.theportlandhospital.com/medical-professionals/find-a-consultant/other/mr-david-floyd/
Simon Eccles, also at the Portland is absolutely the best there is. I know of many children he has performed very sensitive cosmetic work on (after injuries / birth defects etc).
www.theportlandhospital.com/medical-professionals/find-a-consultant/childrens-services/mr-simon-eccles/
I'm not saying your son requires surgery, he probably doesn't. But I wondered if you dragged him up to London for a day out to include a consultation with one of these guys (or someone else specialising in paediatric plastics) you will KNOW if you are doing everything you can to maximise his skin recovery. I would be just like you - I would need to KNOW that I had showed him to the very best and asked what more we could be doing. Perhaps nothing, or perhaps something small - but you will know. Thy will know the best products and best ways to look after the scars as they heal. Other posters are right - this happened just a few months ago. Skin takes much longer to truly repair...jeez, 4 months after my c-section it looked bloody awful (red and raised). A year after that I was delighted (flat, very thin white line).
Even with the passage of time I know that scars can be fixed up and even redone for a much better result (with the right surgeon doing it). Just don't while he is still little and not fearful.
I wish you all the best. You don't deserve to feel what you are. Get better so you can enjoy your little guys. I LOVE boys so much. I am 7 weeks pregnant right now and would just love another little man like you have!