hi there
we are only just transitioning to 1 nap and dd is almost 16 months. You're not unusual, they can be on 2 naps at 18 months sometimes.
With us, the sure-fire sign that dd needed less daytime sleep was the early wakings. She would want to start her day at 5-5.30am. She wouldn't cut her naps naturally, so I had to start waking her earlier from her morning nap. Sounds like this is the case with you.
Just like Emma said, I would start by cutting that morning nap to 45 mins. It might take a while for his body clock to adjust so don't expect to see a change straight away- with us it was a week or more before her night sleep went back to normal.
hope this helps