Unfortunately, I know from personal experience that my 15-month-old could kick the seat in front of hijm – when he was sitting in his car seat, which propped him much further forward. Those flights were not fun – he was too young to talk, so he didn’t understand commands to stop kicking. If I physically held his feet still, which was the only way to get him to stop, he would start wailing.
My son could kick the seat but not with much force. She pushed her seat back right away which cramped his little legs. Fortunately putting the tray table down helped. Not to judge but she really wasn’t a pleasant individual.
I sympathize with that situation. I do want to mention that just because kids are too young to talk (I have two, teenagers now) they are not too young to understand commands. At 15 months they understand quite a bit. I suppose yours was just ignoring you. 
We very rarely flew with the kids when they were young, and not anything like that long: but if there’s any way to get bulkhead seats, do that. A little more legroom, plus there’s no seat in front for him to kick or pull on. The downside is you can’t put stuff under the seat in front of you, so you have to put it in overhead storage.
I wonder if a fabric sleep mask might be good - especially in the middle section, you can’t really isolate him from the rest of the cabin the way you could if he’s in, say, a window seat. Maybe make up a game where this his magic sleepytime mask.
Headphones: might the larger style (vs. earbuds etc.) be a good choice? It would help isolate the ambient noise so he could hear his own music etc. and not be disturbed by people talking.
Heh: just realized, the trip in question was weeks ago. Off to read the rest of the thread to see how you survived!!