I am telling as an example for BAT. He is same in all words if I tell him read this he will spell it rightly but like BAT he will say SAP I am always spending hours to get his homework but he is not understanding the logic he is mugging up . Please help shall I show him to a doctor

Going to a doctor is the next step. It sounds like he's mixing his letters around, could be dyslexia but that is something that's not very easy to diagnose, and usually isn't diagnosed until a child is older.

The doctor will most likely get a hearing test, eye test, and other tests done to determine if it's not one of these issues. If all goes well there he'll likely recommend you to a specialist that will do further testing.

Yes, it's best to go to the doctor to find out what's going on.

Good luck.

Does he know the letter sounds individually? That would be a first step to make sure of. Is his spoken language clear and easily understood? How does he write the words? Can he copy them correctly for the most part? (it is not unusual for five year olds to write some letters backwards)
What has his teacher mentioned about it? Ask her or him for materials you can use at home if need be.

Once you feel he has a good grasp on the individual sounds, have him practice sounding simple words out letter by letter. A bit more challenging is to practice breaking the words down by initial sounds and ending sounds. For example: say the word bat. Now say the word bat without the /b/ sound. Say the word card. Now say the word card without the /d/ sound.

Two fun tools to use so that he can hear himself better are a tape recorder and "phonic phones". These are made of two elbow PVC pipes joined together to look like the receiver of a phone. Kids love being able to listen to how they sound as they are practicing the letter sounds and words.

Certainly bring it up to his doctor as Alty mentioned so that you can try to rule out any potential learning hurdle.