Try asking your Pediatrician.
Goats milk or almond milk is supposed to be a good 'replacement' for whole cows milk or for those with allergies/milk intolerances. BUT, giving "milk" is usually not recommended until 1 years old. And in lieu of "Formula"... I don't know what the 'replacement' would be.
Also, as far as giving him cereal... has it been suggested by the Doctor? Or has he even started "solids" yet? Usually "solids" is not started, per the American Academy of Pediatrics, until 6 months old. Due to the body's development and digestion.
Giving "solids" is NOT a means of just making a baby "full." That is not the purpose.
As far as your baby's intake... yes, at 6 months it is a growth-spurt time. And intake does increase. Each baby is different... but usually at this age, a baby drinks more or more frequently... or even "cluster-feeding" because the appetite of a baby will reflect their every changing needs and growth needs. If I were you... I would offer him feedings on-demand.... not just every 2-3 hours. Breastmilk digests & metabolizes very quickly. He may very well need and be able to drink more than just 3 ounces. How about just giving him a bottle of 6 ounces? A baby will tell you how much they need and "when" you should be upping the ounces a baby can drink per their development and growth. Versus just going by a monthly age.
Also, for the 1st year... iron is an important nutrient for a baby. Or they will lack it... and at 1 years old Doctors usually do a blood-test for Anemia. So.... this is why, with Formula... there is Iron in it, as well as vitamins/DHA's/Omega-3's and minerals etc.
So, if you lose your breastmilk supply... I would definitely ask your Pediatrician what to feed your baby.... so that he does not lack nutrients as well. But don't just switch to some kind of milk replacement without checking with the Doctor.
I know some abhor "Formula." But, again, there are all kinds, and organic or not. Some Moms don't have any other way and their babies are healthy and thriving, with Formula.
The thing is, for proper growth and development... a baby needs a certain intake and caloric intake and nutritional spectrum. Especially the 1st year. It is a building-block time. Have you ever seen a Lactation Consultant to help your baby latch on better, or to ask them HOW you can increase your milk production???? Then, this could help the whole situation...?
I think, try offering him more ounces, more often... again- some babies cluster-feed...which means they even feed every hour. The baby cereal, is just as you said "filling" him up... for fullness. Not nutritionally. At this age typically, they CAN indeed be drinking more ounces.
My kids, went through many spurts of intake increases and cluster-feeding, per their appetite increases and growth.
Maybe increase his feedings/ounces... and perhaps by pumping more often... this can also increase your milk production?
But yes, ask the Pediatrician.
All the best,
Susan