It sounds like your son is "pissed" (pun intended) about sharing his attention with his new baby sister (that I'm sure he didn't ask for). ;-) He's calling out for attention, good or bad, and I'm guessing that he knows that you will have to stop what you're doing to clean up his mess.
I would recommend two things:
1. I know you said you're a single parent, but have you had any one-on-one time with him since sister was born? I suggested that to a friend, and she said it's helped tremendously with her toddler who's a bit jealous (understandably) of baby brother. Could someone watch the baby for 1-2 hours while you have some alone time with your little guy? It's amazing how quickly you'll get to see what is on his mind lately while you two are having fun. Something as simple as getting a scoop of ice cream or playing on the swings with only MOM would be a big deal to him right now. :-)
Also, is dad in the picture at all? Can he have a special "boys day" out where they get to do some fun activity that they don't normally do? Many times there are free activities going on in communities over the weekends that are perfect for toddlers. I get a few email newsletters of what's happening for families around Seattle. Maybe you have something similar there. If not, check the local paper or the city's website.
I'm sure you and your little boy would cherish the time and he would feel listened to and hopefully will act up less. Plus, you can casually talk about things that upset him while you have fun so he doesn't feel like he's in trouble.
2. What are the consequences of him doing this? Does he have to "help" clean it up ? Does he get a time out or a toy taken away? Let him know ahead of time what the consequences are, be consistent and stick with it. Don't spring it on him right after he pees, decide now what the punishment will be and let him know the new rule. Also, make sure you're not giving him a lot of attention during the punishment. You want to focus your attention to his good behavior (even if you feel like you're being over the top) and minimize the attention of the bad behavior.
I wish you all the best. :-)