I agree, you may well be in an UT /OT loop, where he's UT for that first nap, doesn't get enough sleep to be fully restorative, and ends up OT by the end of the day. He's also coming up to the age when many LOs have a bit of an A time leap and get ready to move towards a 4 hour EASY. Not saying he's necessarily ready for it yet, but he may be heading in that direction. Some further info here -
Time to Transition - 3hr, 3.5hr or 4hr EASY.
To fix it, you need to gently increase his first A time, leaving all others the same for a while until you can determine whether an increased A time is helping. Altnough his first A does look good already.... You could try a consistent A time of 1hr 30 for that first nap and see if that helps - maybe even increase it to 1hr 40 if 3 days at 1hr 30 doesn't help.
Other thoughts if increasing A times doesn't help :
1 - This is a classic age for 45 minute naps, which are often developmental. To help with that, you can creep in his room and start patting him gently as he gets to the 33 / 35 minute mark, to help him through the transition. If you're already there when he stirs, you might have more success.
2 - Is his reflux fully under control? Do you think he could be waking from discomfort? If so, no amount of A time tweaking and sshh patting will help. Is he on meds? If so, is he on the correct dose for his age?