Just may be stress from moving. Maybe just use the salt or the "betta-fix". I think that betta fix has salt in it along with tea tree oil. Heat will help, but a more consistant temperature would be best via thermometer and quality aquarium heater. Light bulbs don't know when there is too much or not enough heat. Constant temperature fluctuations is also a stressor. When fish stress they start to loose their protective slime coat. The salt will help with that. When ish are stressed and start to loose their slime coat they are susceptible to disease/parasites. When you transfered him did you also use spring water?

Yes, I did use spring water and the same brand too. I decided to turn off the light as it doesn't seem to be doing anything, and I can't get a larger aquarium to add a heater. He's still not eating sadly and I'm very worried. I'm going to try some freeze dried bloodworms/brime shrimp once they arrive. Oh dear...

A 25 watt heater would be fine. They are rated UP to 5 gallons. Most 2.5 gallon tanks can handle its size, usually 2.5G is 11" x 5" x 11", a small 25W heater is usually 6"-8.5" , If you have a standard glass rectangle aquarium, you can use a less expensive hang on the side type, if it is an acrylic with a fitted hood-light-and/or filter combo combo you may have to get the fully submersible kind, setting you back another $10. If the LFS do not carry them, they are available on-line, I believe PetSmart also sells them in store for about $16-20. More then likely the betta is having trouble from the stress of the move, just like humans age has a roll also. The heater would be a wise investment either way. When he recovers this will aid in prolonging its life, if the worse happens the next fish will have an edge up.

Do you have a test kit? If so, what are the readings for ammonia, nitrite, and nitrate? The first to should be at 0. If they aren't, it is the reason your betta is not eating, and he's probably feeling ill too. Gradually start switching him over to dechlorinated tap water. Bottled water is not good for bettas or any fish long term because it does not have the minerals they need. If used long term, it could thin out the fins and shorten the life span. Don't worry if you have a really high pH, bettas are very adaptable if you transition them slowly.

what have you been feeding him? i tried feeding my first betta flakes, and he wouldn't touch them. he really likes freeze-dried bloodworms, though. they are pretty picky. hope the bloodworms or brine shrimp work! ( i suggest the bloodworms because that's what they eat in the wild)
tighebettalover