I nursed through my whole pregnancy, and sometimes it was a little difficult, but I was really looking forward to tandem nursing. My daughter was 15 months old when we found out we were pregnant. She was down to 1-3 nursing sessions a day at that point, and loving every second of each feeding. There was no way I was going to refuse her one of her favorite things.
There are rarely problems with nursing while pregnant... Especially if mom is eating a healthy diet and getting enough hydration. Often doctors will recommend discontinuing while pregnant, but it's usually because they don't have current information on nursing while pregnant. If a mother has a history of preterm labor, it may be necessary to stop.
The milk does change throughout pregnancy. Supply drops during the second trimester, but with continued BFing, it comes back at the end. It is all colostrum at the end, and many bigger kids don't like it... My daughter didn't mind it though.
One awesome thing about nursing while PG is that once baby is here, the milk factory is already in full swing! New baby will lose very little weight, if any, that they normally lose in the first week or two. My new baby lost only 1 ounce during the first 3 days, then went on to gain 10 ounces in the following week. I also have had less issue with leaking this time, and while I do get engorged, it's so nice to have my older daughter nurse me down instead of using a pump!
I'm also told that tandem nursing helps with jealousy issues. So far, we only have jealousy when little one is nursing and older one is bored and sees us, then she asks for milk and sometimes she gets some; other times I know that little one is really hungry and will need more milk so I tell her I can't nurse her at that time. Usually she understands and doesn't put up a fight.
Yes, the nipples do get sore, but I don't think it was as bad as when I was pregnant the first time and it definitely wasn't as bad as when DD#1 was first born. You can also talk to your child and have him adjust his latch to help mommy feel better.
So far, tandem nursing has been a wonderful experience for me and my daughters. I definitely encourage you to continue, especially if your older child is so attached to it!