He may be marbling but this may be a sign of high ammonia.. Here are some of my concerns..
First off what filter are you using and do you have it prefiltered to avoid him being harmed by the intake tube?
Any time you want to run a tank with a filter and not do weekly 100% cahnges you should own a reliable drops kit for ammonia, nitrite and nitrate. Cycling takes up to 2 months to complete and any number of reasons might harm your biofilter and cause a mini cycle even further down the line. You need to be testing daily with a reliable drops kit for ammonia and nitrite and doing an extra 50% change any time you see either. In addition to this a weekly 50% with siphon is needed to remove poop and other debris from the gravel. It is not enough to just scoop water off the top ever.
First you will see ammonia, then nitrite. Eventually, hopefully, you will see ammonia fall and stay at 0 even after a week of no water changes, and finally nitrite. At this point you will be left with only nitrates after a full week of no changes and these can be kept <20ppm by weekly 50% change with siphon.
You should never replace all filter media in the same month - hopefully you have at least two types of media and three is better. Only replace a max of one a month. Carbon must be replaced monthly. Sponges only need to be replaced every 6-12months but you should swish it in the bucket of old tank water at least a couple times a month during a water change to reduce build up.
All water changes should use same temp water, matched to running tap using the in tank thermometer and the water needs to be premixed with conditioner before adding it to the betta tank. If you don't already have anything, you can use gallon water jugs from the grocery store - rinsed thoroughly in hot water but no chems.
Bettas are tropical fish and must be kept at a temp between 76-82, with 78-80 being ideal (about 25-27C). So your guy is chilly right now.
Feeding blood worms is like feeding potato chips – high in fat and poor nutrition. should look for a good quality pellets whose first two or three ingredients are whole fish, not fish meal or wheat. He should be fed two small meals a day (how many depends on the pellet you pick up) and one fast day a week. Feeding a worm as a treat once a week in replace of pellet meal is fine. Just make sure if they are freeze dried that you thoroughly soak them in a separate cup/container with some tank/treated water for at least 10minutes.
At this point I would change his food, invest in a test kit, and do an extra couple of water changes to be sure.