If the Vet believes the platelet count is an artifact, then a retest might be a logical step. Although in this case it may not matter as the platelet level you gave would still give ample clotting ability to the blood. But the question still remains as to why it's low? The question might automatically be answered with the additional tests.

The Sulfa drug could be the cause, and withdrawal in favor of another antibiotic would reverse the low platelets if the antibiotic is at fault.

Sulfa vs platelets

Note the possibility of kidney inflamation which in itself could cause blood in the urine. Although with good Bun and Creatinine levels that makes for happier kidneys.

The other possibility is that there is internal bleeding inside depleting the platelets.

Presumably your Vet is going to run an ultrasound on the dog as part of the tests. There are various scenarios where castration in addition to major surgery (e.g. removal of a damaged spleen) would probably not cause that much more discomfort to the dog. Or an ultrasound may indicate a congenital structure problem where infections are going to be frequent. In which case castration becomes the only operation to perform. On the other hand if it's a tumor then castration is a secondary issue and possibly no longer important.

You probably need to sit down with your Vet and discuss the scenarios where castration in addition to any other surgery makes sense, and when it might not. A test like an ultrasound may take about 30 minutes, and depending on how mellow your dog is they usually don't have to sedate the puppy. Thus you could wait at the office as they complete the tests, or be called in after they are complete to discuss the results.

Getting a pre-op blood panel will increase the safety per anesthesia. My Vets are very partial to pre-op blood testing. Optional, but I always go along with it. Usually non-geriatric dogs have little problem with anesthesia.

Over all it sounds like you need to get more feed back and discussion from your Vet to address your concerns.