Hey @rosenjcb,
Thank you for opening this thread. Yes you can use the endpoint with the aws sdk for this you can use the InvokeEndpoint method. Java doc
It looks like you are already doing this and there are only a few missing parts i guess.
The Endpoint expects a JSON as HTTP Body and as the error says you are missing the Content-Type: application/json for that.
I have to say i have no JAVA experience at all but i found this on StackOverflow:
InvokeEndpointRequest invokeEndpointRequest = new InvokeEndpointRequest();
invokeEndpointRequest.setContentType("application/x-image");
ByteBuffer buf = ByteBuffer.wrap(image);

invokeEndpointRequest.setBody(buf);
invokeEndpointRequest.setEndpointName(endpointName);
invokeEndpointRequest.setAccept("application/json");

AmazonSageMakerRuntime amazonSageMaker = AmazonSageMakerRuntimeClientBuilder.defaultClient();
InvokeEndpointResult invokeEndpointResult = amazonSageMaker.invokeEndpoint(invokeEndpointRequest)

maybe this helps you crafting your request.
You can also find a example on using the aws sdk with python below
        response = client.invoke_endpoint(
            EndpointName=ENDPOINT_NAME,
            ContentType="application/json",
            Accept="application/json",
            Body=JSON_STRING,
        )