While your cat may be exhibiting symptoms of a UTI, the symptoms you describe also sound like he may have a urethral obstruction, and in a male cat, this is a serious emergency. If your regular vet is closed now and doesn't have emergency hours, you need to bring your cat to the nearest ER vet center, as soon as possible, for treatment.
The antibiotics you have, may help temporarily, but only for the UTI, not the blockage. If you can split the 500 mg. Pen VK down to 31.25 mgs, this dosage should be alright; it can be given 3 times a day (every 8 hours). The proper dosage would be 30 mg., but because you can only get it down to the amount you mentioned, that's the best you can do.
Try your very best to have your cat evaluated and treated by any local vet who may have emergency hours, or an ER vet center in your area, immediately. He may need to be catheterized to drain his bladder, and if this is not done, he will continue to be very uncomfortable, and the toxins can back up in his system, and cause severe illness. Because he's a senior, it's very important to have him seen, now. If you're not sure where there is an ER vet center in your area, and you're locate in the US, try this site:
I hope all will be well with your furry boy, and he's feeling much better, very soon!