A.S.
Some thoughts...
1. If you aren't producing enough, or he's not able to get at the milk because of a poor sucking reflex don't stress. Don't stress. Don't stress. The fact that you've done it for this long deserves kudos! Sometimes we're built for milk, sometimes we're not. I wasn't and although it was tough to face, ultimately my DS and I were much happier when we switched to formula. Much, much happier.
2. Try a Avent sippy cup. They worked for both my kids at 6mos. They actually preferred them since they were easy to drink from.
3. Being underweight may or may not be an issue...again, DS was off the weight chart until he was 2. Now he's a whopping 20%!
GL!