Re: 2nd time around not so easy, he seems to WANT formula!!
Could you have a overactive letdown, or could he have reflux that causes him to fuss at the breast? Arching the back can be a sign of silent reflux.
If he's gaining and having sufficient wet and poopy diapers, I doubt it's a supply issue. Is he gaining? Also, your breasts should not usually feel overly full after your supply has regulated in the first 6 weeks. If they feel full, it means you haven't nursed recently enough. A breast that feels "empty" can continuously be supplying milk to a baby as s/he nurses because milk is always being made if it's being removed from the breast.
I'd suspect it's something other than supply. It can be tough to figure out what, though!
Catherine, tandem nursing mama to Preschooler Girl 9/08 and Toddler Boy 3/11
Breastfeeding for 4 years and counting!