Has Caleb always been low sleep needs? I'm just wondering if even though he might be able to handle long A times sometimes, he can only do that when the deep OT is sorted out?
When did things start going pear shaped with his naps? I can't remember what he used to be like - and why you started transitioning him early, was he a nap refuser?
I don't know your background, but I'm thinking along the same lines as Claire. At 15 mo, I tried several one nap days with my LO, and just like you they were all around 1 hr 10 min to 1.5 hrs. It just wasn't enough to get her through the day. So, we went back to 2 naps, with some of them having to be AP'd (ie. short CN in car on days she wasn't able to do it at home). We then switched to one nap at 17.5 mo, and it started at that same length, but within a couple weeks was lengthening out. I did a set time at 12 pm, like Annette has mentioned and it really did work. Then, my LO started getting her eye teeth (yes, very late at 18 mo), and the one nap started shortening again and OT crept in quick. So, now at 19 mo, we are mostly on one nap days (anywhere from 1.5 hr to 2.5 hr, but with 1.5 hr being the most common, and some still shorter). But, when she wakes early (between 5:30 and 6 am), I still do a morning CN and longer one in the afternoon. I know most people think that's not appropriate for a 19 mo old, but it's the only thing that helps my DD not to get completely OT. If I push her to one nap when she's not ready, she will sleep only 30 min to 1 hr and wake up screaming mad.
So, after all that being said, if it doesn't work to push him out as others suggested, I would try a couple days with a morning CN and a longer afternoon nap, and just see if it helps. You may only need to do one here and there, or you may need to do a few in a row to get him caught up, if you do think he's OT. To me, it just seems that after 3 months, you've waited long enough and it doesn't seem to be lengthening.