Hi, I did it!
I shifted his bedtime from intended 8pm - real 10-10:30pm to 7pm at once. It did not work gradually because the late bedtime was not planned. I just put him down at 6:30pm the first night as for his last nap. When he cried at 7:10pm (he ALWAYS wakes after 40 minutes) I went into the complete dark room without putting on any nightlight, lied down on our bed with him (he is sleeping in his crib in our bedroom) and NURSED him back to sleep which took half an hour. Usually nursing to sleep does not work with him, he's been on EASY since birth and NEVER nursed to sleep. He was wide awake while nursing but finally he relaxed and fell asleep. He woke up after 10 minutes (when the oxytocin wore off), I burped him and nursed him again, anxious that he could wake up for another A-time. It was 7:50pm by then. He woke up at 8:40pm, 9:20 and 10:00. I shush/pat him, nursed and shush/pat at these wakings. I was scared to nurse that often but this was my only chance to keep him in dreamland. He woke up at 1am and at 5am this night which he never did since he was 5 weeks old. (When he fell asleep around 10pm he made it until 5-6am.) At 6am he was awake for the day. The next day I repeated the same thing, he woke up just 3 times instead of 5 before 10pm and made it from the DF at 11pm until 2am and woke up at 6:30 for the day. On day 3 he woke up twice before the DF and at 3am to nurse, 7:20 for the day. Since then it was smooth sailing: I put him down without nursing at 7pm, he ALWAYS wakes up at 7:40, I nurse him back to sleep and DF at 10:40pm (3 hours later). But the best are the effects of that:
He finally is on a routine, not OT by bedtime anymore, does not skip his last nap anymore, no crying in the late afternoon. His naps are still short (45-50 minutes) but I sometimes can extend the third one (1pm-2:30pm) when my toddler DD is napping.
It definetely was the right thing to do. With 12 hours at night he gets out of his chronicle overtiredness and I expect him to get out of his short naps when he'll be ready. I highly recommend to shift bedtime earlier and ensure 12+ hours of nightsleep to anyone dealing with nap problems and OT at bedtime. When it seems impossible to fix the naps give them more sleep at night! It worked for us.
