Using the answer in ModiX's link applied to your code, should result in something like this:
Type genericType = typeof(Deserializer<>);
Type[] typeArgs = { Type.GetType("Person") };
Type deserializerType = genericType.MakeGenericType(typeArgs);
object deserializer = Activator.CreateInstance(deserializerType);
MethodInfo fromFileMethod = deserializerType.GetMethod("FromFile");
fromFileMethod.Invoke(deserializer, new[] { file });
The line
MethodInfo closedMethod = genericMethod.MakeGenericMethod(typeof(Something));
is used if your method is generic, in your case if it would look something like this:
Deserializer<T>.FromFile<T2>(...)