If he didn't start to wake during night (for additional milk) I wouldn't be concerned. He is taking what he needs. It is however possible that he is diminishing his daytime milk feeds and favourite a DF as he is not so hungry during day. But it is very difficult to check when BF so I wouldn't be so much concerned if you ask me. Remember that yield is not a valueable proof for your milk supply and this wouldn't worry me. Is your son taking both sides and finish his milk feeds contended? Do you feel (by time he is feeding) that his DF is bigger than other feeds of the day?
If yes, you may try to shorten slightly his DF to encourage him very gently to increase his milk intake during day.
Regarding solids, 3 meals at this age is fine in my opinion however there are kids who will refuse one of their milk feeds when on 3 meals at this age. But if your soon is eagerly taking every milk feed, I wouldn't be concerned. If he starts to do so, you may stop giving breakfast or dinner. I had times with DS when he was refusing 11am milk so I just cut out breakfast for some time (2 weeks...) and than once again offer it and he was ok. Some people stop dinner (it's wise if your milk feeds during day are postponed because of naps. So if milk feeds are 7, 12, 4 and let say 6:30/7:00 there is no time for proper tea/dinner at 5pm as he may not be yet hungry or he will be hungry but will refuse his BT feed.
Regarding DF, I would probably start to actively making it smaller around 9mo if he doesn't stop it by himself.
Why different size of milk feeds was a sign for us? You know, I perceive my son as a smart beast:). When his milk needs are dropping I cannot do much to change it. When he was drinking less during day, I thought giving him DF was a great idea to make his milk intake bigger. But it was a vicious cycle, as if he took DF, one of his day time milk (or more) were smaller. His overall milk intake during day stayed the same when I dropped DF, he just started to take bigger daytime feeds. So for my son, when his milk needs drop down he alternate which bottle of the day he will refuse or not take to the full. That's happened with DF. He was taking 20ml after day of good feeds, and if he took 200ml it usually meant his morning or pre-noon milk will be smaller.