P.M.
There is a GREAT website about tandem nursing. It helps the older child learn that Mommy's Milk is for both of Mommy's kids, but the new baby needs it first.
If you wean him, it may be the only option you had to calm him and comfort him while he's transitioning to sharing you. I think you should hold off and check out the website on tandem nursing.
She is a real woman, she's really done tandem nursing, and she's more than qualified to give nursing advice.