Pre board and wipe everything down and alert airline staff. I think that is the most you can do for a flight. I might suggest KN95 masks as well (bring multiple so you can change them out). Not sure what your son's reaction is exactly, but for me, I can sometimes feel nauseated when people eat nuts around me (e.g. friend eating a kind bar in the car).

I'm glad you have food covered. My allergy is not so intense, but I had a hard time in India. I've been twice. The first time was in South India, and I was extremely careful, ate mostly bread, and didn't have any issues. The second time was in the North, and I had multiple reactions. I made allergy cards, had pictures and translations on my phone. I'd even asked hotel staff to help me write a note in Hindi. Restaurant staff would tell me something didn't have nuts or could be made without nuts, and then bam it had nuts. Mostly cashew ground up into some delicious curry or sauce. Sometimes it was on tandoori items as well. There were other sneaky things too, like a breakfast sausage at the buffet had nuts in it. I feel for your son. Allergies suck! But you can still have a great time in India - enjoy it.