I can't remember exactly how old she was when she dropped the first nap, but I think she was a similar age to your lo. She has always been at the high end of sleep needs (at 2.5 she sleeps more than my 8-month old!) so I think she was quite late dropping that nap compared with most of her friends.
If he does have a lot of different caregivers, it might be difficult for him to settle into a routine because he's a bit confused. I would think that setting a definite routine and asking everyone to stick to it for a couple of weeks could help him gain a bit of consistency. Maybe once he did that he might catch up from the nearly-OT and then be ready to work towards one nap.
Maybe you could try a short nap at 10.30 (half an hour or so), followed by a long nap at 1.30/2 until 3.30/4. If you think that will leave him with too long an A time before bed, maybe give him a bit longer in the morning, and try pushing the afternoon nap half an hour later.
As far as total sleep for the week goes, it could be worth looking up the average amounts of sleep los need at his age, multiply it by 7 (for the full week) and compare it to a one-week sleep log of your lo. Does that make sense?