There are a handful of causes of urination that has blood in it:
1.) A urinary tract infection
2.) Bladder stones that traumatize the bladder and cause bleeding
3.) A bladder tumor, most likely in a senior pet
4.) Trauma- hit by car, muscle trauma of some kind
Please monitor his urine stream, especially with the recent neutering to ensure that it is not a drip and a healthy stream. It is possible the neuter with his age has inflamed his prostate and this is the result but a recheck exam rectally and urinalysis is advised tomorrow.
Ideally until you are able to get to your veterinarian, please increase water intake. You can do this by supplementing the dry food with water and flavoring the water (maple syrup for example) as well. Increasing the water intake will help "flush" out the bladder and reduce the pain associated with the bleeding.
Again collecting a first morning sample in a tupperware container and refrigerating the sample if more than an hour old to have your vet evaluate the sample will help determine the cause of the blood in the urine.
In addition to a urinalysis there is potential for doing a urine culture to look for bacteria, an X-ray to rule out a bladder stone, and bloodwork to ensure there is not a metabolic cause or kidney issues causing the symptoms
Diagnosing the issue will help determine how best to treat the blood in the urine to get your pet comfortable swiftly
Below is more information on this condition in dogs, called hematuria (blood in urine):
Please let me know if you have any additional questions I can assist with!
If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION. Bonuses are always welcome (I am not compensated in any other way). Thanks!