Hi, welcome to Yorkie Talk
I never had this problem with one of my dogs, but my male nuetered cat was completely blocked from struvite crystals. After his surgery he was kept at the vet hospital for 48 hours with a urinary catheter remaining in to help prevent re-blocking. Was he put on a prescription diet from your vet following his surgery? That helps alot, I kept my cat on it for a month before switching back to his regular food. My cat did have some straining and frequent litter box using for about a week after he came home, but was 100% back to normal after 2 weeks with no reoccurance. These things are common to reoccur after it happens once. It's a bit different with cats as far as the cause (usually diet related, I chocked mine up to the crappy grocery store treats the dh was giving him all the time
). I'd go back to your vet to have him checked since reoccurance is very possible and it's also possible (although if the vet is a good one, I doubt it)that he didn't get them all. I used to be a tech and I've assisted on this surgery many times. It's very delicate and tricky. The other possibility is that he got a UTI from the surgery, but if you gave antibiotics afterwards, it's probably not that, although it is possible. The straining to BM is interesting though, if he was on antibiotics or is on a rx diet, he could just be a little constipated from that.
A trip back to the vet is in order incase it is a reoccurance. I'd say after 7 weeks he should be back to normal.