Monique,
There are several factors that can contribute to a male budgie's cere turning brown. Vitamin/Mineral Deficiency, Hormonal Imbalance and/or Testicular Cancer are among them.
The best course of action is to take your budgie to a qualified Avian Vet for the proper diagnosis of his problem and a good treatment plan.
Avian Vets have special training to determine the cause of symptoms resulting from illness or trauma.
When you rely on unskilled individuals, you delay effective treatment. This can prolong suffering that may be avoidable.
The bird will often require a more intense, prolonged treatment with a poorer chance of full recover than it would have if you seek prompt professional diagnosis and treatment at the first sign of illness.
Having your budgie examined by an Avian Vet allows you to develop a good relationship with the vet in case your bird needs care for an injury or illness in the future. Additionally, it is always helpful for the vet to have a baseline for your bird to refer to should it need future treatment.