Please don't worry.
I flew EasyJet to Italy in May/June with my son on our holiday, I was at my biggest at 24 stone and a size 34, I have my own extender belt because I got so fed up with having to ask for one and the snearing looks you get at times. But they were fine. While I did need the belt, they will have plenty. You don't need to order one before you go and just quietly ask the steward nearest to your seat.
We flew back Ryanair, now these were the tightest fitting seats I've come across out of all of the airlines we've flown with, but that was because we were in bulkhead seats and they have the table in the arm rest, so make the seats narrower, their seats are already narrow so, I still fitted, but it was a very "comfort" fit. No way would I shoot out of my seat if he touched down quickly LOL
I think I paid about £10 for my belt on ebay, I bought it about 4-5 years ago and have used it on various airlines around the world. I am looking forward to the day when I wont need to take it with me, or share my sons drop down table!