Generally, the rule of thumb is that any nap 1.5h+ is considered restorative and LO can handle a normal, full A time afterwards. 30-40ish minute range tends to be OT, and 40ish to under 1.5h tends to be UT.
Try adding only 5-10 minutes more and see if that helps. If his naps suddenly drop to 30 minutes long after trying that new A time, then it was too much.