Mine wasn't BF at that age, but still didn't take to a cup for a while. You might find some successful with a doidy cup. Or a straw cup (Moma straw cups are great). Give it to him to play with in the bath. But really just keep offering and give it time. I think I could get mine to sometimes take a sip around 9 months, but only if I helped. She didn't drink on her own until about 13 months (this is from the straw cup). Then she got it easily after that. 8 months is still young though so I would expect him not to have much interest and to need help potentially for awhile yet.
Ds1 was before and refused a bottle. He had a free flow sippy cup but it did take a while to get the hang of it. He is 2 now and uses a normal cup or a miracle 360 cup if I don't want spillages.
Ds1 is 8 months, mostly because but one bottle a day (I was determined not to have another bottle refuser) he's had a go with his brothers miracle cup and got some out of it. Also got on well with the nuby grip n sip
DD is 11 months and until about 9 months she couldn't figure it out either, tried loads of sippy cups and she'd get water in her mouth then spit it back out! Then one day I offered her the munchkin 360 one (after putting it in the back of the cupboard thinking it was useless) and she picked it up and drank the whole thing So basically, one day it'll all suddenly fall into place. I thought of it as a new skill that can just take a few months of practice.