Not sure what plane you are looking at, but on the United planes with polaris I have travelled on to London there is no way I would take a 3 year old. The seats might look close together on a seat map, but the pods are really quite separated. Even if the plane you are looking at is different, there is always the possibility of an equipment change.
To be honest, I think Premium Economy would be a better option to keep your son closer. If you end up flying from the NorthEast, the flight is too short to get more than a couple hours sleep anyway. If your son is anything like my DS at that age, he would typically stay awake on transatlantic flights until about 30 mins before landing and would then be exhausted and need carrying everywhere.