You have a water quality problem and you have a feeding issue.
Stop feeding the peas.
Your pellet is lower quality. I suggest finding something else. A good quality pellet will have the first two or three ingredients as whole
fish, not fish meal and even worse would be wheat. Of what you're feeding, he should be getting about 3-4 a day, split up into two smaller meals (like two in the am and one or two in the pm). Whatever you feed your betta always needs one fast day a week with no food. Feed one pellet at a time and don't let them sink to the bottom. If they do, fish them out.
He's too hot. It's okay short term (for most fish, though it depends some don't tolerate temps that high at all) but long term it will shorten his life because his metabolism is so high. You should aim between 78-80F. It's also extremely important you use an in tank thermometer to monitor temps (not the stick on and not assuming the heater is running a certain way).Lots of temp jumps are really hard on fish too. It needs to be one stable temp.
Bettas should really be kept in a 2 gallon minimum. The reason is the fast build up of ammonia and the difficulty keeping an even temp. But in something this small you should have been doing 2 water changes a week - one 50% and one 100% including a thorough rinsing of the gravel and all decor.
Here's how to do large water changes without shocking:
To do a water change, use a little cup like a plastic solo cup - this cup must be only for him and have never been used with soap or other chems. Scoop him up in this cup (keep him in the cup about 1/4 full of water - it doesn't need to be much because he won't be in it for long) and leave him in the cup while you change his water. To do the 50% use a turkey baster - dedicated only to him that has never seen soap or chems - and drag it through the gravel and try to suck as much of the poop out as possible, in addition to 50% of the water. Use a thermometer under the running tap to get it to be the same temp as the water that is normally in his tank. When the thermometer says the flowing tap is the right temp, fill back up his tank. At this point, add the conditioner (dose for how much water you change - if you change half the water you add half gallon worth of conditioner, If you do a 100% water change dose for the full gallon change). Float his plastic cup with him in it in the new water. Slowly add a couple tablespoons of the new water into his cup every 10 minutes for at least an hour. Finally, dump him in gently but try to get as little of the old cup water back into the tank as possible. When you do the weekly 100% you will do mostly the same thing except empty his tank fully and rinse everything in it very well under warm water but never use soaps or chemicals. Once it's fully cleaned/rinsed you can refill it and repeat the cup/acclimate phase.
Go ahead and do 100% change now. I suggest using Prime as a conditioner - 3 drops into the current bowl before you change the water, to avoid a potential separate shock/ammonia issue when doing water change after having gone so long. Also add it to the new water at the same rate of 2 drops per gallon or 3 drops for your 1.5g.
I would also look for epsom salt - pure 100% magnesium sulfate with no additives or fragrances of any kind per the ingredients label. Dissolve 1/2 tsp per gallon (so 3/4 tsp in yours) and add slowly over and hour so not to shock. Epsom salts only need to be redosed with water changes, like your conditioner. Keep him in those a week or two. Also fast another 2-3 days or until you see him poop a whole lot and his belly go down. Maybe take a week at the rate you were feeding. Report back on what the poop looks like