Like you, I had a lot of questions concerning solid foods and continuing to breastfeed. My daughter is now 8 months old (she is still breastfed), and we weave in the solid pretty well. We started her on solids a few weeks before she turned 6 months old. As some other moms suggested, at this stage, it's really just practice and introduction to new flavors, aromas, and textures. Most of what I have read, suggests that you only need to offer solids once a day as a SUPPLEMENT only if your baby is still breastfeeding. This should be the case till aorund 7 months or so, then step it up to two meals a day. As many of us nursing mothers know, breastfed babies often have no schedule, so be prepared to make adjustments. If you continue to breastfeed as you have been, the best timing is to nurse him on command as he wants, and when it works into your daily schedule (around breakfast or lunch or supper), wait about an hour or hour and half after he nurses and the offer solids. This way he is starting to get hungry again and may be more open to it. At around 6 months, a nursing baby doesn't really need more than a few tablespoons of solids (fruit mixed with cereals tend to work best as you are starting off). 2 tablespoons equals about an ounce.
As he gets more used to different foods and wants to eat more, up it 2-3 ounces of food (maybe a few different things) twice a day. I htink if you try and make it consistent (I know it's tough) you'll find yourself getting into a schedule, or at least close to one.
Just as a side note, baby foods are quick and easy to make at home by purreing them in a small food processor and freezing them in ice cube trays (each cue is about 1 ounce). Plus, this can allow you to defrost just what you need, offer your baby more of a variety at one sitting, easily defrost more if he wants more, and save some money!!!
I know there is a lot of info here, but I hope it helps!!