He only needs 1 nap a day. If he will not sleep he at least needs extreme quiet time.If he gets up at 7:00, then have him play very active from 10:00- 11:30. This means outside running around climbing, jumping hopping.. Riding a trike..
Bring him inside and wipe him down so he can cool off. Feed him a quiet lunch speak softly . Make sure your home is cooling down.. No TV... Then nap right after lunch.. This should be a quiet story with a fan blowing on him.. You may want to cover the windows in his room with something to block out the light. One mom said she uses black trash bags.. Then just like when you were teaching him to sleep at night, every time he gets up, walk him back to his bed and just say, its time to sleep. EVERY TIME. You can also tell him he has to stay in his room in his bed. Do not mention sleeping.. Let him look at books or play quietly.
Then when he wakes up it is a tiny snack if he needs it and then running and playing then errands, then get ready for dinner, bath, quiet time.. Get the room ready again.. fan, darkened not a lot going on.. And bed.. If your neighborhood is loud with children outside playing and he can hear that.. You may want to play some music or purchase a sound machine.
It does take a lot of effort in the beginning.. This is not unusual. Your little boy is wanting to be where all of the fun is and he thinks that is with you entertaining him.. If it looks, sounds and feels like not much is going on, he would much rather hear a good story, be comfy in bed and sleeping.
I am sending you patience and a good nights sleep.