well, basically, its a good idea to feed him when hes hungry, instead of putting him on a "schedule". he and you listen to his hunger cues; this makes a healthy attitude about food.
always do the nursing FIRST.
its probably not that you cant keep up - remember that breastmilk is demand and supply. so the more he nurses, the more milk you will make. if you dont let him nurse often or long enough, your body wont know to make more milk. hes 6 months old; its growth spurt time. its likely that he does need to nurse VERY often just to up the milk supply. do what you can to nurse first, and nurse often. if you want to continue breastfeeding, dont suppliment unless and until there is a serious issue with his weight gain, or energy levels. you will know if hes not getting enough because he will not look healthy. just listen to his needs and your instincts. dont let anyone else tell you that your milk isnt good enough; its likely that it is you just need some support!
its a good idea, if you are wanting to continue nursing, to contact a local or nearby la leche league leader. www.llli.org. they will be able to help you specifically, directly, and give you real, mom experience.
with my son i literally nursed on demand night and day, and never had a problem. so i believe that is the best way to do it; but it doesnt work for everyone, and i dont have enough details about you to know if its a true milk supply issue (but its not as common as people think it is).
anyway, la leche league is the place to go.
OTHERWISE: if you dont care, you've still done a great job nursing this far! nurse when you want, your body will make the amount of milk that your nursing demand tells it to. you can nurse once or twice a day, and your body will make that amount, as long as you do it every day at those times. ;)
but yeah, really, you arent limited to rice cereal, especially if you are doing nursing, because breastmilk gives your baby EVERYTHING he needs through age one without needing any of the cereal. but feed him whatever you want to feed him, even soft tiny portions of what the rest of the family eats at meals or whatever. :) anything you give him he will at least play with, which is just as good of an experience as eating it.
but like i said, dont worry as much about a routine, as you do feeding him when hes hungry, nursing/bottle first, solids after. and stop when he shows signs of being full. :) thats the best you can do!