Hum, I'm going to try to cover all the bases here......You say the DR has done tests, but you didn't say for what so I'm going to assume for UTI, bladder and kidney infectins. Did he also do a CBC blood test to see if his white blood cells were up? This would be a sign of infection in the body.
He is only 3 1/2 so he could be testing and or falling off the wagon of being potty trained. From what I've read, it's not uncommon for them to regress once in a while. My son is 3 and started having accidents at night out of the blue for a few weeks and then they stopped just as fast! Also, you might want to reconsider the standing thing when he is going potty. I still make my son sit down only for the fact that he has no aim yet!! And sometimes it doesn't come out in a straight stream either. He is only allowed to stand when he is outside at a tree! lol A friend of mine has a 4yr old and every time he is at my house I have a pee covered toilet, all the way up to the top of the seat because he can't aim and he does this at home too. Drives me nuts!!
You didn't say where your son was when he peed while talking to dad, but you should make him help clean up when he does do this. If all else fails, you might want to take him to a urologist and have him looked over to make sure there isn't somthing more going on with him.