My baby won't nap...

My LO is 3 months old and since he was born we let him choose his schedule as I ebf on demand. Since he was 3 weeks old he has been sleeping through the night 9:30 pm to 7:30 am. Lately though he occasionally adds a night feeding but I think that's due to teething as he wakes up crying his "pain cry". (He had two teeth cut through this past week) But he won't nap longer than a 20 min stretch during the day and I'm lucky if he'll have more than two short cat naps. At one point he did have a 2-3 hour nap but that was short lived. He gets so tired during the day but won't fall asleep! Any suggestions on how to get this baby to take a nap?