I'd try a longer gap between nap and bed. 5 hours is quite short at this age. You want a longer one after nap before bed than the morning one.
I could be that he is just not getting tired out enough in the mornings to get a nap. I know on days like today when it's 45 outside we take a short walk and then spend most of our morning inside. Any time that happens my daughter won't take a nap but we do quite time. I put her in her room same as nap time but with some cuddly animals and a few books. She will lay there and "read"/talk to her animals. I think break in a quite place is just as helpful.
My son has been doing this for a few months now. It's exhausting but you kind of have to roll with it! Just keep to your mostly usual routine but try to make sure you're doing something stimulating/physical enough in the morning to tire him out for his nap, and if he doesn't want to nap, just leave him to have a quiet time like @Sarah said so he calmly unwinds somewhat and you can have a little break too. If he's extending his wake windows, you might need to cap the nap a little so it doesn't make bedtime too late. My son's wake windows are often over 6 hours, which is a struggle to fit in a nap but he certainly still needs one most days! 😅
I would push nap to 12:30/1, cap the nap at 2 hours, then push bedtime back as well. I would also try cutting back on the benadryl as it could be negatively impacting his day sleep
It is early to drop the nap but some babies do drop it this early. Just roll with it.
I think I’ve heard that Benadryl can have the opposite effect for a small group of people and can actually make them more wound-up. So if this correlates with his nap skipping then maybe that’s what could be happening… just a thought.
14 hours of sleep per day is the very high end of average for 1-2 year olds. It’s definitely possible his sleep needs are just decreasing.