B.V.
I would encourage you to try to reintroduce nursing, even if you supplement with formula. The social and bonding benefits of this close, special contact between mother and child are wonderful, even if nursing can't supply all his nutritional needs. If you do try to keep nursing, your milk supply might increase and start to meet your sons needs.
If that is not successful and you have to pump to continue breastfeeding it is SO beneficial to your son. Even though you think your milk is watery, it still has SO many benefits for your baby - like natural immunities and compounds that can't be duplicated by formula. I would discuss with your son's dr. before deciding to quit. Most will say that even a little breastmilk is MUCH better than none.
Regardless of what you decide, I would recommend LaLeche League as a great resource on everything related to breatfeeding and weaning in a loving (and least uncomfortable for you) way. http://www.llli.org//. Their book - 'The Womanly Art of Breastfeeding' is great and gave me lots of ideas and encouragement when nursing and/or pumping is tough.
Good luck!!!