Bruce has stopped eating

My son's betta, Bruce, has stopped eating, and he was hiding in the bottom of his tank. He normally swims around, eats like a pig, and creates huge bubble nests. Sunday is fast day for the fishies, but I noticed old food floating in the water presumably from Saturday night. (My son feeds him and didn't watch to make sure he ate, I guess.) I am guessing this is from stress, but I wanted to run it by the experts.

I have a 2 year old who has gotten in the habit of playing in Bruce's tank. I am planning to move it to a higher location since telling my other son to keep his doors shut hasn't worked. Thursday, he got into it twice - once right after I had cleaned it and again later that evening. I recleaned it the first time because he had thrown a candy cane into it. At least that time Bruce was still in his cup getting acclimated to temp. The next time, he reached in and took out the thermometer which he subsequently broke. I didn't see anything that he had thrown into the tank, so I left it alone.

When I noticed the change in behavior last night, I did a 100% water change and tested everything my kit will test. It was all okay. I tossed the gravel in case the little one threw any of those little metal beads from the thermometer in there that I could not see. (He seemed to perk up a tad when I got him into fresh water, so I wanted to make sure I eliminated the possibility of a toxin in the water that I could not test for.)

This morning he still refuses to eat and is very mopey. Other than that, he does not show any signs of illness that I can see. I am planning to buy him a new heater that is adjustable. He has had presets to this point, but they do not keep the water as warm as I'd like with him being in the basement. What else can I do for him?

Housing
What size is your tank? 3g
What temperature is your tank? 75
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? TopFin Betta Bits
How often do you feed your betta fish? 2 pellets, twice a day

Maintenance
How often do you perform a water change? weekly
What percentage of the water do you change when you perform a water change? 100%
What type of additives do you add to the water when you perform a water change? Betta Bowl or Stress Coat dechlorinator

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

Ammonia: 0
Nitrite: 0
Nitrate: 0
pH: 8.2
Hardness: My kit does not test hardness or alkalinity, but we have a water softener
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? little paler, fins not held as wide as normal (not clamped to his body, but he's not holding them as wide open as he usually does)
How has your betta fish's behavior changed? somewhat lethargic, hides at the bottom of the tank, not eating
When did you start noticing the symptoms? last night
Have you started treating your fish? If so, how? 100% water change, have him floating in a cup in the tank I am cycling to keep him warm until I can get him a new heater today.
Does your fish have any history of being ill? No
How old is your fish (approximately)? He's not very old. He was pretty small when we bought him, and we have only had him since the end of November.