Most major airlines have bassinets available for little ones.
Not any more they don't.
Our 6 month old was a nightmare on her first flight. She wouldn't sleep because everything around her was so interesting, and she couldn't calm down. So she just wriggled and strugled the entire time. We had to play with her the entire time, which was very demanding when you are knackered and there's no space to do anything really fun.
I'd say definitely book a seat if you can afford it, otherwise you might get a screaming thing elbowing you in the adams apple the entire time whilst you try to rock him and sing songs etc. If you get a plane with say 4 or 5 seats in the middle and you book three of them, chances are you'll get the entire middle block to yourself if the flight's not jammed full. That way you at least stand a chance. But it is very hard. If you can go a couple of months earlier, then do.. below say 5-6 months you might geta way with it depending on how alert your kid is.
And tbh the pressure change is the very least of your worries.
And to all those who moan about kids on flights - bugger off. I'm not stopping my kid seeing her grandparents just for a few hours of your mild inconvenience.