Hi- I started my DD on the 3 hr routine at 8 wks. I fed her on demand prior to that. she was very difficult to manage at feedings, and wanted to eat at all random intervals- and then was tired and fussy and wouldn't eat and it was just a mess, so this whole routine thing truly revolutionized my entire life as a mother. She is a new baby- so happy and full because she eats full feedings- now about 5.5 oz at a time (she'a over 3 mo. now) and feeds normally one side at each feeding.
about your questions: He actually won't be eating less milk- he'll eat more at each feeding, the feedings are just further apart. And this way, he'll put on weight because he'll be eating the rich hind milk- mine stopped fussing once I started this because she was fuller after each feeding and easily can go until the next 3 hour mark. what I did do because she was still young was feed on demand before bed...so she would "tank up" and cluster untill she was full enough to sleep through till about 3:30 (even if I did the DF)- but she quickly started to sleep until 7... SO- she would feed nearly every hour from 4 until 8 or 9. She gradually started to drop the clusters. Now she doesn't at all. she's perfectly on every 3 hours. it's WONDERFUL. my breasts have really caught on too-
I would start to log your day with him...see if he has any natural patterns and work from that to start. try to lengthen the times between feedings. It's really hard at first, but at 7-8 wks is the perfect time to start (you get the best of both worlds- on demand for the newborn, and routine for the baby and for your sanity). my dd picked up on it pretty quickly. It took about a week though. The sleeping was the hardest for her, but she got that as well. Now we're perfectly on a 3 hour starting at 7am every day. Like I said, she's a different baby now that I feed her every three hours. I would deiffinitely start him on it now- because if you do wait longer, it makes it harder for him to catch on and break any habits that he has. He's young enough now that you can pretty much mold his daily routine. let me know if you any more questions....