My baby is currently 5 months old and has been ...

My baby is currently 5 months old and has been exclusively breastfed up until now. I had planned to breastfeed for as long as possible but lately LO seems to be losing interest. He used to feed A LOT! And would happily take the boob at any time but over the past couple of weeks, even when feed time is due, he arches his back and squirms away and doesn't want anything. But if I give him expressed milk in a bottle he will guzzle it down without any issues. I would much prefer to express and feed him that way instead of using formula but I've been struggling to get enough milk for him. I thought I could try giving him formula perhaps once a day but I have no idea which formula to buy. There are so many to choose from and I'm feeling pretty lost. I also heard that giving babies formula before bed can help them sleep through the night because it takes longer to digest so keeps them full for longer. Is this true? LO is still waking up 2-3 times in the night for feeds and I'm willing to try anything to get him to sleep through.