I think that if he can handle a 2 1/2 hr A time in the morning you shoudl try to get his second A time to 3 hours and build from there.
so try to gradually stretch him to being awake until a nap of closer to 1:30 or 2pm (if woke at 11) and see if he will still do his 1.5 - 2 hr nap. then i woudl aim to have him up from the end of that nap until bedtime (which you could move earlier until he gets used to the new routine and eaisly handles an A time of 3-4 hours+ for last A cycle) a way you can get him through (aiming maybe for 6:30 and then gradually increasing and pushing out bedtime as he seems ready) is to take him for a walk in the stroller during his usual catnap window - so maybe give him his milk after the nap and dinner at around 5 and then take him for a walk - if he falls asleep fine and if not at least he has had little active stimulation (just stim. from nature...), then do bath and bedtime routine from 5:45/6pm
he might surprise you - have you tried lengthening that 2nd A time before?? if he manages to nap until 3-4pm you might find he can make it to 7pm bedtime if you have a low key after dinner period (if at home can sit and cuddle for a bit, play classical music, read to him...) and take about 45min for bath, drressing, feed, bed... HTH