I haven't flown him with him as a lap infant on longhaul but he's been fine on the two short haul trips we've done, we've found out from BA that we'd need a travel restraint if I booked him his own seat, between Canada's rules (canadian manufacture only and that he'd need to rearface in the car by age and weight) vs BA's rules (forward facing) and the UK's lack of market for airline suitable car seats its looking like lap infant is our only option, we're debating now whether we fly premium economy with BA over economy for that bit of extra space

My daughter travelled well as a lap infant at the same age so I guess i'm hoping for fingers crossed!