Im 3 months post an umbilical hernia repair with mesh.i've had a small seroma and discharge for over a month.he has cauterized it &discharge is brown?

Infection: It sounds like an infection and like the mesh may need to be removed.

Old blood: It is very common to have some bleeding under the skin after an operation. That can collect there and is called a hematoma. Old blood turns a brownish color as it resolves. If it smells foul, there could be an infection. It should be healed by now, bit sometimes tales a little longer. Stick with you're surgeon for regular visits until healed, or inquire about another opinion.