Congratulations on your pregnancy!
Your breastmilk has and still will have a lot of nutrients for your little one. It still provides him with proteins, fats and immunity and actually the older he gets, the more antibodies can be found in bm. It is of course up to you to decide if you want to continue but as long as your pregnancy is healthy you may continue nursing. The best thing you can do is to make an informed choice by learning about benefits of extended nursing and/or tandem nursing. You can find a lot of info here:
http://www.kellymom.com/bf/tandem/index.htmlSome women continue nursing their first one during pregnancy even though they report discomfort while breastfeeding, others who do, don't experience any nipple soreness. Sometimes milk changes its taste and a toddler decides it is not good for him anymore, sometimes milk dries up, especially in the 3rd trimester.
Also, if you wean now, he won't probably remember his nursing experience when you get your new baby. If you continue to nurse him and decide to tandem nurse, he may actually be fond of a new baby as his needs (nursing needs) will still be met by you and he won't resent a new one.
There are lots of things you can find out on this subject and only you can make decision that is the best for your family. Good luck!!!