My sister son 5 month older, and doing their heart surgery heart is off and problems on brain due to lack oxygen. So what are we doing now?

Depends. The options depend on how severe the brain injury was, time with low oxygen etc. There are many possible treatments depending on those issues. Please talk with your child's cardiologist to learn more about your specific issues.
Heart surgery. From what you have written - it seems that he was deprived of oxygen to the brain and has sustained some brain damage. There is not mush you can do except wait and see if he wakes up. Talk to your doctors.
Talk to MD. I am not sure what surgery you were planning, but if there is a problem with the brain the surgeon is probably going to want this to resolve before operating. Check with the surgeon for a more complete explanation.