Hello,I am so sorry your little one is having trouble.
I would love to help I just have a few questions so I can better assist you.
What color are his gums?
Is he eating ?
Any increase in frequency of urination?
Thank you sorry for the inconvenience.
The straining and blood in the urine are most commonly due to urinary tract infection, bladder stones or crystals in the urine.
Read more here
Blood in the urine can also indicate a bleeding disorder. This is most likely due to infection and not related to the surgery, however a urinalysis will be needed to be done by the vet to rule out or confirm this possibility.
With this happening so soon after surgery I would keep an close eye on him until you can have him seen by the vet. Watch for any additional signs of bleeding such as pale gums, bruising, or small red spots on the gums, chest, or stomach.
Try to encourage drinking as much as possible to help flush the bladder. 3-4 Tablespoons of Cranberry juice will help lower the PH in the bladder and make him more comfortable until he is seen.
I hope all goes well