I agree with everything already said. I nursed DS throughout my entire pregnancy with DD. In fact they both are still nursing
It can be overwhelming at times but it is so sweet at times also. Honestly, I'd just play it by ear and see how you do. Most kids your son's age will wean during the pregnancy anyway so if that happens then you don't have to force anything on him. If he doesn't you can use that time to establish limits so he's used to them when the baby comes.
Good luck to you either way!