Goodness, it sounds like you've been through quite a lot in just 15 weeks. It must be terribly frustrating and demoralising for this to happen just as you felt you had cracked it.
I wonder if your ds could be expressing frustration at the speed of milk flow at the breast? Milk in a bottle tends to sort of dribble out farily easily, whereas to get milk to flow readily from the breast, babies have to work a little harder initially to trigger the reflex that makes the milk come out more quickly. Their behaviour at the breast can be more fussy until this 'let-down' of milk starts. Some babies settle down and take one full 'let-down' of milk and take the edge off their hunger, but then seem impatient to wait to stimulate another burst of milk flow. Compared with bottlefeeding which is continuous, the milk flow in breastfeeding comes in waves. If you feel that this might be a possibility, then there are a number of approaches that could help. Breast compressions are a really helpful technique that some mums find helpful in this sort of situation. Others try using some of these tips to encourage the milk to flow faster more quickly. Frustratingly, the hormones needed to get the milk flowing quickly are inhibited by feelings of anxiety and stress, so it can be all too easy to get into a cycle, where a mum feels nervous about settling down to feed, because they are anticipating the same problem as last time, and then they feel tense, which inhibits the hormones and compounds the problem. Anything that helps you both to feel more relaxed at feeding times is worth trying.
You say that you are also concerned about his weight gain and feel that it is slow. That must make it even more difficult and worrying for you that breastfeeding isn't going as well as it was. Can you say a bit more about his weight gain and how much he's putting on? Is your HV concerned? It's actually quite common for breastfed babies to slow down in the rate they gain weight at around this age, and many do grow slower than the lines in the red book charts would suggest from around 3-4 months onwards. Charts that better reflect the growth of breastfed babies are available here.
You say you are expressing and also topping up after every feed now. I wonder if you might want to consider limiting the top ups to specific times of day, rather than every feed. So perhaps give a full bottle at set times, rather than topping up every time you breastfeed. There are 2 reasons to suggest this. Your baby could come to expect a bottle after every breastfeed, and this might actually compound the issues you're already having with keeping him at the breast (although breast compressions may well help). Some babies become reluctant to work as hard to extract milk from the breast when they know that a bottle will be available to them with a steady flow of milk as soon as they stop. This can become a spiral where mums end up giving more and more top up, and as others have said, it sounds as if you are topping up quite a bit at the moment.
You also say that you are concerned that this recent change may be affecting your supply. It's true that if you find yourself offering more and more formula, then your milk supply will decrease as the amount of milk your baby removes from your breasts decreases. However, by this stage your supply will be well established and flexible, so it's very possible that a period of longer and more frequent feeding would bring it right back up again. Mums can establish a milk supply from practically nothing -called relactation, so it is definitely not impossible.
There's some more info here that might help you to work through some of the other reasons he could be fussing during breastfeeding and seem reluctant to latch on well. Often mums find that at around this age their baby has grown quite a bit and that perhaps they may still be holding them in the same way that they did as a newborn, when perhaps their baby is much longer than they were and their head is significantly bigger. Sometimes going right back to first principles and adjusting the way they hold their baby and bring them onto the breast can be enormously helpful. There are some clear photographs here that may jog your memory. The key is a big wide open mouth with head tilted right back, coming onto the breast chin first and top lip just sliding over the nipple. for newborns, it's often said to start nose to nipple, but as babies grow they might need to start a bit further away than this because their mouths are bigger, so more like forehead to nipple, with their head starting in your cleavage. Other mums find that their baby has simply become more efficient at feeding and actually doesn't want to feed for as long any more. You could perhaps experiment with simply allowing him to feed whenever he wants and for as long or short a time as he wants. It could be that he wants lots and lots of very short feeds at the moment.
If he won't feed, then expressing is the best way to keep up your supply. To maintain a supply when a baby's not feeding, you'd probably need to pump as often as he would feed, so every 2-3hrs. Some women get on better with hand expressing, or find that hand expressing and/or breast massage before pumping can help to indrease their output. As with breastfeeding, stress and anxiety make expressing more difficult, so anything that helps you to relazx or distracts you from the pump may be helpful. cup of tea, shoulder or foot massage from dh/dp, watching TV or chatting rather than staring at the pump etc etc.
I agree with Tiktok that spending lots of time just cuddling could be really helpful. Some people really enjoy having a bath with their baby too, with no pressure to feed.
I also agree that you would almost certainly benefit from some 1-2-1 support in person or by phone. Are there any breastfeeding group snear you that you could visit (would your HV know?). If not, you could try the National Breastfeeding Helpline on 0844 20 909 20.
Do let us know how you're getting on.