Wow. My daughter is 25 months, and still nursing, so obviously I don't have first hand experience with this! But, as usual, that won't stop me from telling you what I think!
It's really okay to go with your instincts, to listen to your own discomfort. If you feel funny or creepy about nursing him, then I'd say, don't nurse him. Despite having someone attached to them for two years, my breasts are still my own, and I do plan to take possession of them again someday. Hooray to you for doing it, mama!
I also think it's okay to be gentle, but honest, with your child. Sometimes, kids are very sensitive and intuitive. If you tell him how you feel, very simply and with lots of love, he might understand.
I also think it's a really good time to get him prepared for you to nurse the new baby. One thing lots of preschoolers love, is to see film footage of baby animals nursing. And most kids do like the role of older sibling, especially when they are involved with baby's care.
Best of wishes to your family.