Hello and welcome to BW forums
If he is sleeping 1.5hrs without assistance (ie after wind down you put him down he falls to sleep and sleeps the full nap without further help) after a shorter A time then I would continue with the shorter A time until it doesn't work any more. You might need to cap a nap if it starts getting too late for BT but if you've already been running this day time routine then you will have already resolved that.
Weaning from the rocking and replacing with shush/pat or doing a little rocking in the cot with a firm hand will help to sleep train for naps. Try to work towards having hands off before he goes to sleep and when he can do that you work on leaving the room before he goes to sleep.
If/when you do feel the A time needs to extend just extend by 15 mins for a couple of days and then a further 15 mins if needed rather than making a big jump. The smaller increases in A time give Lo a chance to get used to the longer time without getting too OT.
Signs LO is ready for a longer A time can be short UT (under tired) naps of 40 min or resistance being put down at nap time.
hope this helps