I think it could be OT or UT. Him waking up happy could have meant that that is all the sleep he needs, except that he falls back asleep after that.
When he gets cranky, you can walk around with him or do finger rhymes..anything low key really. With mine, walking around really helps. This is just until he adjusts to the new time. Also, since you are aiming for 2 hours, you can start with your routine to get him down 5/10 minutes before, giving him some time to settle to sleep. A time is eyes open to eyes closed. With us we go in 5 minutes before, change diaper, swaddle, sing song while closing the curtains and then lay her down.
As to extending the nap, after a while, OT would add up and it will affect bedtime even. If you can extend a nap without much trouble, you should. You never know - one day he might surprise you by transitioning himself. That is why it is important to get the timings right -- right amount of activity and right amount of tiredness.