Same thing happened with me, but Kali was about 20 months when I got pregnant with Skye.
I let her nurse until I was four months away from my due date, then I put a stop to it. I was too sore and grouchy, and I couldn't imagine tandem nursing, so I allowed plenty of time for her to get over nursing before I started bfing her brother. It really wasn't that hard... I used distraction techniques, and only cut out a nursing at a time, and was very gradual about it.
HOWEVER, you have a 13 month old... do you use lots of Lansinoh? It may get easier in your second trimester, as your body adjusts to the newness of being pregnant again. If you don't want to tandem nurse, maybe you could plan on gradually cutting back until he's completely weaned by the time you're due in, say, two or three months.