Your boys fins are rather clamped which is a sign that he is not happy however I can understand your situation and am so glad that your are getting him a 10 gallon tank with a heater and filter. Apart from water quality the main other problem associated with small tanks is being able to heat them. As long as you keep his water nice and clean and keep him as warm as possible (Maybe you can wrap his Jar in a blanket at night to help insulate him) them I am sure he will be fine until you can get him a more suitable home.
Don't feel bad and you are doing your best for him right now and the most important thing is that you are working towards getting him in better conditions with a lovely new tank.
Most of us start out keeping bettas in far less than ideal conditions due to lack of knowledge or being fed incorrect information, this forum is a great way to learn about successful betta keeping. You should not have to worry about people feeling hate towards you, you are working towards giving your boy a better life and that is something to be proud of..... we all have to start somewhere and joining a forum like this is a great way to learn about correct betta care.
Keep us updated on your lovely boy and when you do buy your 10 gallon and get it all set up be sure to post some pics on here we would love to see it