Kelly - you're right. Although alot are on 1 nap, not all are comfortbale staying awake till midday. Mine used to do this during the 2-1 transition so I wouldn't say its habitual waking, she was waking and crying from overtiredness.
At 15 months, Isabella's routine was: 6-6.30 awake; 9am (nap for 30-45mins); 1pm (nap for 1.5hrs); 6.30pm bedtime. She did transition around 15-16months but certainly on the days when she was tired in the morning, I used to give her 2 naps.
Have you tried maybe bringing his bedtime forward, to 7pm??? An earlier beddtime solves nightwakings for us 99% of the time.
If he is finding it hard to stay away then let him have the 2 naps. But if on the days you cannot get a morning nap in and he has a nap say around 11ish , then you should bring the bedtime forward - more like 6-6.30pm. If you keep him awake till his usual bedtime, he is awake for way too long and that leads to overtiredness (& hence restless sleep at night).
Try earlier bedtime & let me know how you go
Layla