Looks like we're back on track!
Last night he only gave one 'aahh' at 10.30pm, woke up at 6.15am!
Routine is 1 nap during the day of 2-3h. The better the night, the shorter the nap. We never let him sleep longer than 3h during the day. Today he woke up by himself after 2,5h nap.
We try to let him stay awake untill at least 11am, if possible even later than that.