First, I'm surprised your doc put him on formula to supplement! My dd is very petite (at 8 months was 13.5 pounds), my first doctor was always concerned about her weight so told me to stop bfing, give formula and start solids at 3 months

. Instead of following this, I found a new ped. She wants me to continue bfing, NOT add formula, and to continue the df for now and will check dd's weight monthly. She's not worried as dh and I are slim but just wants to monitor the weight. My dd started solids at 6 mths as well.
If your ds is on target developmentally and alert I wouldn't worry too much. I'm thinking that perhaps your supply has been impacted by the formula supplements. As your ds has grown your body hasn't made more milk to keep up with demands because of the supplements. Also, formula flows feely from a bottle, he may be frustrated that he has to work for the breast milk. 10 minutes could be long enough, many los become efficient feeders, my lo nurses for 10-15, 20 if she's very sleepy. Temporarily, I would nurse more often to increase the supply, especially with the big 6 mth gs around the corner then stretch out the feeds to be on 4 hour EASY, especially for when you introduce solids.
As for feeding before sleep- I've always offered a top up before bed for my dd and she takes it (even though it's less time than the EASY). She pretty much 1/2 falls asleep at the end of the feed but then she wakes when I take her to her room and put her in her crib so technically, she's not 100% asleep from the feed.