2 naps are not necessarily a problem, but sometimes we have early wakes or late nights because hes not tired enough. I think more of a structure would be good for both of us, regardless of 1 or 2 naps but I'm stuck where to start
I'd keep 2 naps. Your posts don't give any reasons to suggest he's ready for 1- nap days.
What he's lacking is routine and structure, as you've suggested yourself. So i'd start with that.
As a guide for where to begin with a routine, it's generally ideal if each awake window is longer than the last. It's called the 234 rule- so 2h from waking to Nap 1, 3h from Nap 1 to Nap 2 and 4h from Nap 2 to Bedtime. This is just a guide though, not a set in stone rule. It might be that 225 works better for you, or 244, or whatever works for you.
Include meal times in your routine. So have dinner at the same time every day. Likewise lunch. No distinction between weekend and weekday for toddlers, the ideal routine will be identical 365 days a year. Get up the same time, sleep at the same time, eat at the same time, every day, week in week out.
So it starts with a wake up time. Not an "fingers crossed he sleep a to 7am" wake up. But a realistic one. So if he usually wakes at 6am, then 6am it is every day. So that means setting an alarm (just while you're establishing a routine, not long term) and waking baby up at 6am if not already awake.
This routined wake up, gives you a by-the-clock time for Nap 1, with a 2h awake window so 8am.
Then have another wake-by time. 1h30m sleep is good, so that gives you 9.30am. If baby isn't already by then, wake him up.
By having a finite end time for Nap 1, it gives you a more predictable time for Nap 2. 3h awake time give a you 12.30.
I'd aim for 2h for this one, but your routine says 1h. It might be that earlier timing will give you a longer nap. Or maybe aiming for 1h30m nap is better. Give it a week and see how it goes, but I'd hope for 2h - so set a wake-by time of 2.30pm.
Then 4h to bedtime means ideally it's 6.30pm bed (which works about right with a 6am wake up). It might be that once you are more routined, you can then start pushing the whole routine an hour later, so 7am wake and 7.30pm bed.
Or it might be that 345 suits your DS better than 234. So 6am wake. Nap 1 9am-10.30 (wake him up if not awake). Nap 2 2.30pm-4.30pm (wake him up). 9.30pm bed (Or earlier if Nap 2 was shorter).
It's about finding something that works and then being very consistent. Every day, no matter what, nap and eat at the same times, always.