Bloating has other symptoms involved, a rounded belly after eating is normal and healthy.. you just don't want it to become too large. You can fast for a day if you wish, but not necessary unless he is having trouble swimming.
It depends on the brand of food you feed, on how much to give to your betta. As well as how old the betta is.
They should be fed at least twice a day, or more if possible- with high quality food that have meat/fish meal (or actual seafood) as the first ingredient.
Some pellets are rather small, and you could feed more of. Some are larger and you would want to feed less. Just keep in mind that feeding once a day you tend to either over feed, or under feed.. and neither is ideal.
If the pellets are the larger ones that are about the size of their eyes, then 2-3 pellets per
meal, twice a day is recommended.
If the pellets are smaller (i.e. the Omega One betta buffet pellets) then you can easily feed anywhere from 5 to 8 per meal, twice a day.
Again, that is for an adult betta (over 1.5 inches body wise- or approx a year old. Keep in mind majority of the bettas you purchase at retailers are only around 3-4 months of age).
Younger bettas will require a smaller portion, but more meals per day then an adult.
If the food is low quality- aka cheap- then the chance of bloating and becoming constipated is higher. But it doesn't mean it will happen as long as you are feeding a good amount.
You will also want to make sure you give them a variety in their diet- high quality pellets are good staples for common keepers, Omega One, New Life Spectrum seem to be the top choices for most keepers. They have good quality ingredients.. I personally feed Omega One for when it's time to feed pellets (or for my very very picky eaters who will only eat pellets (a disadvantage of taking in older bettas sometimes)). I feed anywhere between 4-8 per meal depending on which fish it is- I have all sizes and age ranges.
Some flake food is actually very nutritious- the Omega One betta flakes are actually a bit better then the pellets, just have to know how much to feed and be prepared to remove any uneaten immediately after feeding time.
Frozen foods can be okay, just make sure to get the better brands, as sometimes the freezing process can actually take out a lot of nutrition and makes the food empty calories.
Freeze dried has protein, but lacks in everything else and is considered empty calories as well.. good for a treat if you wish, but I never really gotten into feed FD with mine personally. I have two different types, but they have collected dust over the past year since I prefer healthier treats.
Live foods are also great, and free... all you need is a decent sized bucket of sorts and a place to put it outside.. fill it with water, twigs, leafs, maybe some grass and you have a nursery for mosquito and daphnia- use a small fish net or a shrimp net and scoop out the larvae, quick rinse under some water and your fish will be in heaven. Natural instinct to eat them will come into play.. fun to watch them chase the food around.
You can also raise your own live food- the fry of live bearer fish (guppies, platies, mollies, etc) can be fed to larger bettas as well for good nutrition, and it gives you a chance to fill the fry's guts with nutritious food that will in turn end up in your betta.
Also there are recipes for homemade food that is awesome to feed, if you wish I can find the posting for it here.
I hope this helps you some.. again, 6-8 pellets a day isn't bad, but make sure it's spread between 2+ feedings, and they are higher quality. Then you won't have to worry about bloating. :)
True bloat you will see difficulty in swimming, tipping (similar to sbd), you may also have constipation that goes with it, etc.
*The 1.5 inch was not meant to say that a betta is an adult at that size, but rather the betta is large enough to feed larger quantities, and less frequent then the younger/smaller bettas*