My son was like that. Big boy too 95th-98th percentiles.
For the 1st year of life, per our Pediatrician, breastmilk/formula is the PRIMARY source of nutrition for a baby, not solids and not other liquids. And to feed on-demand.
Nurse BEFORE solids. If you nurse after solids, he will be too full.
Your son is 6 months, they get distracted while nursing, or they teethe and it makes it harder to nurse so they "seem" to reject the nipple... or they get tired of nursing. Because nursing takes work... a lot of mouth/tongue coordination and waiting for the let-down etc. AND you said your milk is dwindling. SO, that probably plays into it too.
BUT, 6 months old is also a growth-spurt period in which they normally increase their intake.
My son, was that way as I said. He had a GINORMOUS appetite too. But, when teething, he hated nursing. My daughter was that way too. But I had lots of milk. If teething, it is a phase. They will return. But if your milk is not increasing, they may not want to nurse. OR, it is that your body is adjusting to his lessening nursing, so it is producing less.
Or, he may be self weaning. But again, its important that they have a certain intake, or breastmilk/formula. Ask your pediatrician ho much he should be drinking.
Next, and this of often criticized... but maybe you need to supplement with Formula. My son, at that age and older, would nurse from BOTH breasts still until they were empty, nusring for about 1/2 hour... and he'd still want MORE. So, per our Pediatrician, I supplemented with Formula. He would then drink an entire bottle.
Then, by 1 year old, he pretty much self-weaned himself from breast.
Some say that breastfeeding for 6 months is enough. Others say longer is best.
I would check with your Pediatrician. Ultimately.
All the best,
Susan