I have been in a similar postition recently right down to DD starting to walk! She is on one nap which is usually 1˝ hours and then sleeps from 5.30/6.00pm until past 8am but I decided to wake her by 7.30am now so she can take her nap at midday and still get to bed at a decent hour.
Recently although I was being diligent about her morning A time I let the PM one slip (she was sometimes awake 5.5hrs) and we started getting crying at the halfway point of her nap, I thought nothing of it but then she started to take ages to settle for bed at night and was waking on her own earlier, around 7am and then bam 2 nights of NWs, I realise she was accumalating OT from the long A times before bed, luckily it was nipped in the bud but I would take your DS's screaming as an early sign of OT, especially if he used to sleep 2˝hrs. It could be different for you but if he wakes after 45 mins and doesn't go back to sleep then maybe less than 5 hrs A time would be good just to keep the OT monster at bay
