It can be hard to judge based on behavior as sometimes animals act "fine" even though they are suffering. When my kitten was at his sickest, he still played some. When he got well, I noticed he had more energy, but at the time it wasn't a very noticable difference.
If when you monitor him you notice him struggling to pee (circling and/or digging a lot, crying, getting in and out of the box, etc.) he needs to go to the vet. It's a good idea to bring a urine sample too if you can, though this can be hard to do. One trick is to cover the box with plastic wrap, but if he's not using the box then forget it. My kitty liked to go on hard surfaces when he was sick so I could just use a syringe from the vet to get a sample. Also, wherever he goes, look to see if the urine is dark yellow or brown in color (this indicates highly concentrated urine and/or blood in urine). Also try to feel it and see if it feels grainy, which would indicate crystals. The crystals are very painful but can also block the urethra which calls for an emergency trip to the vet.
Hopefully it is behavioral, but if you suspect at all that it isn't, he really needs to go in because if he does have a UTI or a blockage it is a serious situtation that needs to be addressed immediately.
Amy