You could try one nap a day, i know there are plenty of lo's who have made the switch at this age. My advice would be to shorten the am nap to half an hour and see if that helps first. the danger with this transition is going too fast too soon and ending up with OT.
The first A time is quite short at his age. I would aim for more like 3.5 hours, then a half hour nap and then he should still go down around the same time as your DD.
A - 7am
S - 10.30-11am
S - 2-whenever
Bed - 7.15pm
How about that?
becky x