Clamping down could be for a few different reasons; could be teething (my ds did this a few times when teething), could be he's not hungry at the time, could be he is way more interested in doing something else for the minute. As for his 5 min feeds, he could be just more efficient at nursing so doesn't need as long. With that said, I would perhaps wait 15 min and then try again. My ds often would nurse for a very short time, pull off, and fight me to get back on the floor so I would wait and try again and he would bf the next time (or time after that).
As for lengthening the time of feeds in the day, I would hold off and I say that because he's taking 2 night feeds which could possibly mean that he's not getting enough in the day. I would wait until he is totally over being sick first to see if you can get on your regular routine and then see about stretching the feeds in the day. Is he on solids yet? This could also impact the quantity of milk he is taking.
He could also be fussing and not wanting to bf because he knows the milk comes faster and is easier for him to take from the bottle he's getting. By when did you want to have him weaned off bfing? Depending on when, if you want to go much longer than I would maybe just bf in the day and change one of the night feeds to the bottle of formula, otherwise if it's sooner than change another bf from the day to another bottle of formula.
