I'm sorry you have been concerned about Sherlock! I agree that he is likely having a urinary issue.
The erections are likely due to discomfort. He may have a bladder infection, crystals in his urine, bladder stones, etc. It's a good sign that Sherlock is eating and drinking, but I am concerned he may be at least partially urinary obstructed. If he becomes completely obstructed and cannot pass urine at all, then it is a true emergency.
You need to get Sherlock into a local vet today to have his bladder palpated for possible obstruction and for a urinalysis. Once the vet determines what all is going on with Sherlock they can then provide proper treatment for him based on the clinical exam findings.
In the meantime completely clean out Sherlock's litter box so you can closely monitor his ability to pass urine. If he gets in his litter box but does not manage to urinate or only produces a few tiny drops, then you need to get him into a local vet right away. Continue encouraging water consumption to help keep his urinary system flushed out. Adding a little tuna juice to his water will hopefully entice him to drink more. I hope Sherlock is all right and that he is feeling better for you soon!
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