I can tell you my recommendations but I will also tell you this: Everyone has their own opinions on how to keep these lovely fish. You need to decide what works best for you and your fish.
Since it is a 1 gal, it actually has less than 1 gal of water with all the gravel, plants, decorations and other things you put in there. Keep that in mind. Because of that, it would probably be best to change the water every day. 100%
I use Prime by Seachem for my conditioner. I love this stuff. One bottle will last you quite a while! I use it in all my tanks including my breeding tanks.
Plants - I favor anubias, water wisteria, and java moss. I have all three in my tanks. Low light. I don't add and fertilizers or do anything special for them and I use regular gravel. I tend to leave my plants floating with some java moss floating and some resting on the bottom.
Tank size - Personally, I would upgrade to a 2.5 or bigger with him so you can have a little more room for adding things and decorating.
Food - I use a few different foods. I have two kinds of pellets (one being New Life Spectrum) that I use. One brand is a bigger pellet which I feed to only 3 of my bettas (Odin, Hannibal, and Uther). The rest get the NLS pellets. I only give freeze dried foods to Hannibal. You have to be careful because freeze dried foods can cause bloat. I also keep the following frozen foods on hand: bloodworms, mosquito larvae, brine shrimp, and daphnia. Daphnia is good to have on hand in case of bloat as it works as a natural laxative. A varied diet is good for your betta.
As MrV said, you have to be careful with Double Tails. I feed my Sholto once a day with one fast day a week and he looks great, very active.
As far as his color goes, he may or may not brighten. Some are dark. He may or may not grow. I have had some grow since getting them and others that have not. We got Hannibal from a PetSmart and he has grown since coming home with us.
Also, some filters are too strong for bettas. This one may have just been too much for him in a small space. You don't need one in a 1 gal really since you need to do such frequent water changes anyway.