He's very pretty! 5.5 is the perfect size for a betta, good job for sticking with your instincts! In an unfiltered tank, I would do two water changes per week: one 50% and one 100%. A filter is definitely not necessary, but I highly recommend one, especially in anything 5 gallons or larger. Those 100% water changes get annoying in larger tanks. Five gallons is big enough to hold a cycle, so if you got a filter, once your tank cycled, you'd only have to do one 50% water change a week--much less hassle! Again, this isn't necessary, just a suggestion because it'll be a lot less work for you!
Bettas love a lot of crowding, so as long as he has room to maneuver and swim (and it looks like it does), I'm sure he's perfectly happy with a lot of decor. Just keep on eye on the plastic plants...some of them can rip fins.
I don't know what could be causing the cloudiness, sorry. Does his tummy look round right after you feed, and then go down to normal a few hours later? Or does it look round even several hours after you feed him? A round belly right after feeding is perfectly normal, and just means he's full. But if his belly remains round for a while after feeding, you may want to cut him back one pellet every day and see if it helps. It's a fine line between feeding enough and feeding too much; it always takes me a lot of trial and error to figure out how much to feed my fish.
As long as he is otherwise acting healthy, don't worry if he doesn't flare. My male won't flare at his reflection at all; the only way I can get him to flare is when I let him see my female. Some bettas are just more docile and laid back than others.
Congratulations on your fish and your setup, and good for you for saving him!