For babies any sleep for more than 15 minutes is considered a nap. He may do this whilst he is feeding, but still it would be considered a nap. If he is gaining weight well you need not be worried.
As far as the crying is concerned, he may be colicky. Burp after each and every feed. Even then if the crying continues, kindly consult with your pediatrician
Next Steps
u can try blacking out the room he is sleeping in and also cancel out any sounds or use white noise if it helps him sleep longer