[aselvarial]

Ok, first off sweetheart, a .5 gallon tank is really too small for him.
Unless you are doing twice a day water changes, he is probably stressed AND sick. It's hard to keep water a good temperature without a heater, but wrapping a towel all the way around it will help to slow the temperature variations in the water.
Can you give us a little more information about the fish and tank so ppl can more accurately help?
Housing
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?

Food
What type of food do you feed your betta fish?
How often do you feed your betta fish?

Maintenance
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?

 

[aselvarial]

Lol, not my list of questions, its what they ask for in disease. In half a gallon, ammonia builds up fast. Daily water changes would be best. Also, if you plan to upgrade him this weekend, here is what its said on here is the minimum. A 2 gallon tank and a heater. I loved the aqueon mini bow 2.5 but it was on sale for $20 when I bought it. Until you upgrade him, do daily water changes and try to match the the water temp. Also flakes can cause bloating and are harder to get out. Stick to his pellets for now. Use a turkey baster to clean out any leftover food (or in the case of the small tank, feed before water change) He will probably stay stressed until you can upgrade him, but doing daily water changes with a good water conditioner (which I'm assuming you already have) will at least keep down the ammonia

 

[aselvarial]

If you have a petsmart, they have the top fin 5.5 gallon starter kit for $28. All you need to add to it is a heater and a thermometer. I have the 29 gallon version of this tank and am pretty happy with it. If you have space, the Grreat Choice 10 gallon starter kit for $30 is pretty nice. Just add heater. I have the 10 gallon and am pretty pleased with it. If space is a premium, the Top Fin Aquascene 3.5 gallon is on sale for $23. An heater ran me $10-$15, so if you need to stay around $30, that's your bet.
BUT, and here is the best part, if you get the 10, you can always make a divider and get a second betta. And since you already have the filter, heater, food, water conditioner, and space, all he costs is the price of himself and a divider. And trust me, bettas are like potato chips. It's really hard to stop after one (i currently have 5!)

 

[aselvarial]

And, no, not really much way to keep him in a .5 gallon and not have him be stressed. It's pretty much the equivalent of a human in a closet. Yes, they can survive, but it's not pretty. Try to do two 75% water changes daily, if you are worried about moving him constantly. Or 1 100% change and 1 50% change daily. That should help keep the ammonia from stressing him out too badly.
And flakes, well, they swell, and bettas are prone to constipation. Not to mention flakes are harder to clean up than pellets.
Btw, I totally understand. My first betta purchase was a whim for my 4 year old. We bought it a half gallon tank. He stayed in it for less than 24 hours before I researched it and found out how much work it would take to keep them alive in such a small tank. He moved to a 2.5 gallon Aqueon Minibow (it was on sale), the next day, and the when I acquired a second, we got a 29 gallon tank (on craigslist, an AWESOME resource if you live near a city), divided it, and then as I got 3, 4, and 5, we got a 10 gallon and divided it. You live, you learn, and do the best you can.

 

[aselvarial]

Relax, 1.5 gallons is totally doable. No one thinks you are trying to kill him as you did come here. Next step, heater. Then put a wtb ad on craigslist and check your local classifieds. Ppl offload tanks all the time. Worst comes to worst, subscribe to Amazon Prime (still free for students I believe) and order a tank off Amazon. Concentrate on a heater now, and you can easily have a happy healthy betta in 1.5 gallons. Just check with those on here who have tanks that size for water change schedules.