Are there any dental clinics can do it for my 6 years old son? His first molars is getting inflammation and 1/3 crown was decayed. I hope it can be repaired because his molars on the other side has been extracted. Thanks!

If they are adult molars he will need a root canal if the tooth can be saved. Look for a specialist called a pedodontist who specializes in treating children only.
It sounds like you need to look closely at the BIG picture... what is your child eating and drinking that is causing this kind of decay??

Sound like you are neglecting your kids health and setting him up for dentures at age 18

There is no excuse for things to have gotten that far