Based on his history, we do have a few concerns. Now the most common cause for blood in the urine of a young dog is a bladder infection. That said, dogs with bladder infections usually produce small volumes of urine frequently, strain, and are in constant discomfort (just as people are with cystitis). Therefore, this doesn't quite fit with his situation. And in that case, our main worries here would be urine crystal issues and bladder stones. Both can cause intermittent severe discomfort and trigger bleeding within the bladder that can appear as blood in the urine.
In this case, I am glad that he is feeling better, be we do want to tread with care. Therefore, I would strongly advise at least collecting a urine sample (free catch, not off the ground) to submit for analysis. This can be checked for signs of infection but also examined under the microscope for crystals. Otherwise, if he is due a booster soon, it'd be ideal to move that appointment up so this can be checked. That way his vet can just make sure there are no lurking bladder stones (which could get bigger over time and potentially cause more persistent signs and even require surgery to remove).
Overall, these would be our main concerns for his discomfort this weekend and that bit of blood. Therefore, we'd be best to check a urine sample and make sure there are no underlying issues lurking that we can head off to avoid any more signs like these.
All the best to you both,