Is my cockatiel sick? Should I take him to the vet?

This is my first time ever owning a bird of any sort so the fact that they can hide their sicknesses so well gives me terrible anxiety when I have to leave over the weekend. Recently I've noticed some changes in my birds behavior, his name is Wrin. He is very fluffed up a lot of the time, more than usual and there are a suspicious amount of feathers in his cage in one spot with other tail fathers (I think) littered around the rest of the cage. A couple weeks ago I got really freaked out because he was just kinda sitting there but my dad convinced me he was probably fine because my cousins bird was doing the same thing and when she took her to the vet they just said she was molting and she would be good in a few days. But I've also noticed a multiple of other things that are concerning like his balance is off somehow, hes usually pretty grumpy with me but over the last few days hes been letting me do a lot of things he normally doesn't like touching his feet, touching his wings, and getting in his face. He would normally hiss at me. I've noticed his poop is kinda brown and stringy? Idk if that's normal or just because of the food hes eating but it used to be green, but we did have to change his food since he ran out and we didn't have time to drive to the place we normally get his food. And lastly he sneezes a lot throughout the day I read that, that could just be that it's to dry so I don't know. He looks like he's shivering a lot when its not cold too. I'm hoping someone with more experience with owning a bird is able to tell me what to do because like I said this is my first time ever owning a bird and it's scary as **** to know that they can hide their sicknesses so well. I'm only 15 and I have to go to my other parents house on the weekends so I do have to leave him here for the weekends and every other week for the summer so I can't always be here with him and my parents can't pay as close attention as I would like them to since we also have a 7 month old baby in the house which is understandable. Please I don't know what to do and this is very scary. (I'm also aware his cage looks bare, he has a lot of new toys coming in the mail dont worry he just chewed up all the other ones)