C.T.
Don't stress. It most definitely could be the teething causing a loss of appetite. Also the more you increase his food intake the less formula he will need. Although not always with a growing boy. You need to take your cues from him. If he is showing disinterest in bottles then he is probably not hungry. It took me a month to figure out why my daughter was fussing at certain bottle feedings. All of a sudden, within one day, she went from taking 6 six oz bottles down to four. She refused and I was pushing her to eat as previous. She began taking fewer bottles with more formula. If he is refusing certain bottles at a certain time try cutting out that bottle. If you are worried add a few more oz to the next.
Also you need to look at his food/formula intake over a period of a few days (2-3) not from meal to meal. As long as he continues to gain weight don't worry.
We also started our daughter on cereal/rice at 6 mon. We began at dinner. Always offer the food before the bottle. Eventually the food will replace the formula... don't be surprised if they fill up on food and take less of the bottle. This is the goal. We started making smaller bottles as to not waste. He will quickly go from eating a few tablespoons at dinner to eating a bowl full of cereal mixed with fruit or veggies three times a day. I would say within about 6 weeks. Get him on a feeding schedule, breakfast, lunch and dinner, just like normal. With two snacks in between. We do breakfast, dry snack (cheerios, crackers, et.) lunch, nap, snack (fruit, cheese, yogurt, ect.) dinner. This will become easier with the more foods you begin to introduce. www.wholesomebabyfood.com is a good reference site. Also if you read get BabyFacts it dispels a lot of the myths and worries that mothers have. The food before bed is most definitely a myth. Feed him dinner when you eat.