K.C.
Can you define "fussiness?" Like, when is he fussy - during nursing, after? Bottles, because of the way they fit a child's mouth, often don't cause a breastfed baby to suck as much air and keep his tongue down, so he WOULD be less fussy.
Is he fussier when you nurse sitting up versus laying down? If he is, it could be he can't handle your fast flow and is getting over-filled too fast.
Does he nurse at the breast for about 1 or 2 minutes, pop off and fuss, then try to relatch? If he does, he is probably not latching correctly or putting his tongue to the roof of his mouth. Either way, he is getting frustrated with trying to nurse and is probably sucking air, which will make his tummy hurt.
Let me know if you need help - I am a peer breastfeeding counselor and have worked with moms all over the country (and about 10 here on mamapedia who are all nursing well now!)
Peace :)