I flew when Jovie was 5 months and put her in the HAB and fed her at take off and she slept until landing. On the way back she cried at take off and wouldn't feed or take the dummy but it lasted a few minutes & then she settled. DH had to help cover me up while she was fidgeting but it worked okay.

One flight from the US to Australia when Matilda was 18 months she vomited all over me during the landing for our stop over in NZ & I didn't have a change of top so had to sit in a wet vomit smelling top, so I recommend an extra t-shirt for you packed as well

So I guess the most important things would be:
breast or bottle whatever method you are using
dummy if you are using one
extra wraps for vomits (put one between you & Mason jic)
t-shirt for you
1-2 soft toys for Mason
then your normal nappy bag situation, nappies, wipes, change of outfit for him, etc etc