At 17 months old, you do NOT "have to" transition him to 1 nap. This is NOT a "rule." At this age, many still do benefit with 2 naps, and need it. Do not just make an "absolute" only 1 nap a day routine. Some days he will still need 2 naps. And personally, I still think he does need 2 naps. He is not responding well, to not napping.
There is NO "formula" to this. Each child is different. You need to go according to YOUR child, and his "cues."
In other words, each day will NOT be the same nap "schedule."
For example: my son is now 2.5 years old. From the time he was 6 months old, he has taken 2 naps a day, and still does at his current age. BUT, there are days that he will not nap in the morning. But, I still go through our pre-nap routine, and I wind him down, and I put him in his crib. He KNOWS the routine like auto-pilot by now. Then he usually amuses himself in his crib for a time for about 15-20 minutes, then he falls asleep, and naps for 2 hours. BUT, if he does not end up falling asleep, then I take him out. But I wait, for about 1/2 hour.... and if he's not sleeping by then, I take him out. THEN, in the afternoon... after I pick up my daughter from school (for me I have to time his naps according to my daughter's school schedule and pick up times)... then he takes his nap at about 3:00, and he naps for about 2 hours minimum.
But the thing is... I KNOW when he is tired, and I KNOW his sleepy cues... and I ALWAYS put him to nap, REGULARLY.
If he does not nap, he is NOT happy and it affects his whole demeanor for the entire day.
I did for a time, "experiment" and omitted his morning nap and then went to toddler groups at that time. BUT, he did not do well and would be tired, and got clingy and he would tell me he wanted to go home. So, taking away his morning nap, for him, did not work out well.
So, my son still takes 2 naps a day, about 80% of the time.
But, this is my son. Each child is different. Some kids do fine with just 1 nap a day, consolidated. On weekends, I sometimes just put him down for 1 nap (because I don't have to worry about timing his naps per my daughter's school schedule and pick up times), and so on weekends I will put him to nap anywhere from about 11:00-1:00... and he will nap for about 2-3 hours.
At night, no matter how many naps he took, and no matter what time he took his nap or for how long... he still goes to bed at night at about 8:00-8:30. And he does fine with this "schedule" we have.
But, my son is very regular, and at about every 3-4 hours or so, he gets tired. But this is for HIS age, right now.
For me, I keep VERY regular and consistent about his naps. I am always home when it is his nap time. I don't force him to nap... but I KNOW his patterns and tired cues. Each day is different...so you have to be flexible about it.
For you, your son STILL seems to need 2 naps. To me. He still gets tired. But, his "resistance" right now, is probably due to his changing development, and also when they are hitting milestones, they sleep ability gets tweaked. Its a phase. So, keep this in mind. No matter what "phase" my son went through, I flowed with it. BUT, I KNEW when he was tired and when he would not nap, or would. AND I knew that after the irregular phases, he would go back to his normal nap patterns. And he did.
Your son is barely making it by 12:00 without a nap. So this is an obvious cue to you, that making him skip a morning nap is not good. He NEEDS it.
And OFTEN, when a child is "over-tired" and too tired, it actually makes it HARDER for them to fall asleep, and harder for them to sleep well and have a quality sleep. So, making him "tired" on purpose for so long... could actually back-fire. As you see...and it is interfering with his "ability" to sleep/nap.
And then, by 3:30-4:00 he is miserable and as you said, your afternoons are miserable, and it is because he is just TOO over-tired and TOO bent out of shape by then.
Before any nap, you need to have a "wind-down" time. For me, it takes my kids about 1/2 hour to wind them down first, BEFORE they actually go to nap and I put them down. And, if you keep them "stimulated" all the way up until nap "time"...they will NOT be able to unwind before nap.
All the best,
Susan