I think the other moms are right in saying that it's just a phase. Boys don't typically get bladder infections unless there's an abnormality in their "plumbing". Their ureter is far longer than in a girl and it takes an awful lot for bacteria to get up that far into the bladder to cause an infection.
I would say if he can hold it when he's outside, maybe the others are right in saying he's doing it for your attention and just let him go potty on his own. Set it up so the toilet's fully accessible to him alone and tell him to use it whenever he feels like it as he's such a big boy now.
However, just to be on the safe side, you can either send a first morning urine to the pedi for a check or they sell UTI dipsticks in regular pharmacies now. You can check his first morning urine and it can tell you if there's anything to be alarmed about, although they're probably not 100% accurate (although what is?). Although bladder infections aren't common, urethritis is a little more common in boys than girls. So, it's worth getting it checked.
Good luck!