I was only making sure that it is from his cues that 2 hours works for you, not an *idea* that this *should* work.
You might not be able to change how easy he is to put down, but I'm sure you can try a few things to make it easier, and stretching 3 naps into 2 can be a good start. It sounds like he is able to sleep for long enough stretches, and at 5 months old might be able to be active enough to stretch 2 hours a bit longer. Transitions can be difficult, though. You can often tell if they need a transition, though, when they start becoming difficult to put down in the evenings, or start waking up like you say he's doing now.
Transitions from 3 naps to 2, or 2 naps to one can take some patience. Move the morning nap later by, say, 10 minutes per day. He's old enough to be engaged in his surroundings now-- you might be able to distract him for just a few minutes without being overtired. Then push bedtime later by 10 minutes each day. Don't postpone the 2nd nap. Let him go down when he's ready. Try not to be in the car (if he falls asleep there) before naptime until he is firmly established in a 2-nap routine, if you can at all help it.
I wouldn't assume that you can fix this personality entirely, though. Eventually you will run out of naps to eliminate and he still might not be easy to ease into sleep. Establish habits of soothing-time before bed. Dim the house lights slightly. Keep the TV off.
Remember also that we are having the lightest evenings of the year right now. That can translate into difficult sleep transitions for everybody.
I know my 2nd daughter was very difficult to put down, and with a difficult older sister, we pretty much gave up naps for everybody when she was 18 months old. She *did* need them, but dd2 was phasing out of them, and doing both together was *impossible* and it was less tortuous for me if I trained myself to not expect a nap from them or a nap or break for me.
Hang in there. Nothing ever stays the same with raising little ones.
"Let me see you stripped down to the bone. Let me hear you speaking just for me."