Hi StaceyPaige, welcome to the forum. That's really great that you are doing your best to improve Hubert's living conditions.
The sad truth is unless you know and trust your pet store, you just can't trust anything the employees tell you, especially about bettas.
First, does Hubert rest on the sand a lot or does he seem to only do it for a short time, like he may be sleeping? If he stays on the sand all day, then he's still very sick and lethargic but if he only rests there every now and then, he should be okay. Were you only able to find aquarium sand at your pet store? I used CaribSea Naturals sand for my aquariums - it's meant for freshwater aquariums. If you're sure you got the salt out, there's probably no reason for concern but if you want to be certain, you can take a sample of your water to your pet store and have them test it for saltwater.
He's got a cute little face and pretty coloring. Once he's back to full health, his coloring should become more vibrant. Is he eating all right? He looks a little thin. The weird coloring around his gills is probably ammonia burns which should heal with aquarium salt treatment. And the best way to keep Hubert on the road to recovery and healthy is to keep his water as clean as possible. With his 6.6g (looks like a Fluval Chi tank?), you can change 50% of his water every week. With sand, it's easier to use a turkey baster to suck the waste out; a gravel vac will just stir the sand up too much. Every other week, change about 75% of his water.
I hope Hubert continues to get better and keep us posted.