I'm very sorry to hear about the passing of your mother-in-law, but very happy that your family has taken=in her little baby! He's an adorable little Green Cheek Conure, and yes, he's a Yellow-Sided Green Cheek Conure...So his actual species is a Green Cheek Conure, and his color/mutation is Yellow-Sided. The Crimson Bellied Conure is a totally different species from the Green Cheek Conure, but they are very closely related and look very similar in both size and appearance (much like the Black Capped Conure, people think they are a type of Green Cheek Conure because they look so much like them, but they are not)...
I too have a male Yellow-Sided Green Cheek Conure, his name is Bowie, I brought him home from his breeder when he was 12 weeks old, and he's now about 2 years and 3 months old...and I love him dearly. I can tell you that Green Cheeks are extremely intelligent, and are often called "Velcro Birds" because of how closely they typically bond with one particular person, and then want to be with that person/on that person all the time. They are very loving, snuggly, cuddly little birds, and also tend to become very protective of that one person that they bond closely with. They do tend to be a bit nippy, but they typically don't actually "bite" people; they are also very stubborn and will not give up doing something if they get it in their head that they want to do it, lol...
****One thing that you might want to consider trying to do is to make sure he's getting more out-of-cage-time than it sounds like he is, as you mentioned that you "let him out every day for a walk around"...In all honesty, parrots like the Green Cheek Conure, who are so intelligent and who crave so much attention, really do need to be given at least 4-5 hours minimum of out-of-cage-time every single day, and that's the minimum, more is actually better. I'd suggest that you either buy or build him some type of play-stand, T-stand, Manzanita or Java Wood perch, play-gym, etc. if you don't already have one for him (PVC piping/fittings are extremely cheap and easy to make stands/gyms out of, you need a $10 PVC pipe-cutter hand-tool, and then you just buy the fittings to make whatever design you want to, and then simply wrap the pipe that is meant for the bird to walk on with different colored Vet-Wrap from Tractor Supply); you can make a huge PVC play-gym yourself for under $20, so you save a fortune from buying some type of stand/perch/gym from a pet shop. It's important that he get lots and lots of out-of-cage-time, flying time, and interaction time, because if he's inside of his cage all day long, every day, and only allowed out of it for an hour or so a day, he will become bored, and that's when the self-mutilation/feather-destructive behaviors start, along with screaming/squawking...Just do your best to try to let him out of his cage whenever you can; for instance, if I'm home, then that means all 4 of my larger parrots are out of their cages, period. It's the first thing I do when I walk through the door, I let both of my dogs out into the back yard, and then I let all 4 birds out. And all 4 of my larger parrots were trained/learned over-time that when their cage doors open, they automatically fly to their play-stands/play-gym, and that's "their space". They each have their own play-stand/T-stand, and then we have a massive PVC play-gym as well. So even though their cages are all located in the main-room of my house, where I spend most of my time when I'm home (it's my living room in my house, where I watch TV, read, play Playstation, play guitar, etc.), they are still all let out of their cages as soon as I get home. They either go right to their stands, or they go to their cage-tops to play. And each stand has lots of toys to both play with and chew on, and the big gym is covered with different types of toys. So even if I'm not directly interacting with them, they still are not locked inside of their cages, and they know that they need to stay in their areas and are very good at entertaining themselves...I had to teach/train them, but it doesn't take long at all for a bird to "take ownership" of their cage, their play-stands, etc. That's "their space" and that's where they automatically go. And this makes it much easier for you too, because he won't be flying all over the house and getting into things...Or, you can simply let him out and put him on your shoulder, and allow im to just hang-out with you whenever you're doing things like watching TV, on the computer reading, etc. Either way, try to make sure that he is getting at least 4-5 hours each and every day out of his cage, and that he has tons of different types of toys inside and outside of his cage, and that you rotate new toys in each month. Otherwise he will eventually become very bored...
Hopefully he's on a healthy, low-fat, varied diet as well...I don't know what your mother-in-law fed him, whether his staple diet is pellets or a seed-mix, but if it's a seed-mix, please make sure that it's not a seed-mix that contains any sunflower seeds, no peanuts or other nuts, and no corn...These types of seed-mixes are extremely high in fat, do not provide the bird with the nutrition he needs to be healthy, and are the main reason that birds in captivity die young from Fatty Liver Disease. If his main staple is a pellet then that's great...and of course lots of fresh veggies and dark, leafy greens...
Don't ever hesitate to ask any questions you ever have, it's always better to ask than to guess!!!
