Fair warning that I typed a lot of words because I want to be comprehensive and to help you with any questions, but I'll tell you things that you need to know when starting a model
Because you haven't built a model in such a long time, I'm assuming that you don't have the tools that us "modern-day" modelers have. But that's okay, because the tools are not really expensive just to start modeling.
I also have no experience in making airline models, but I do have experience in cars and ww2 era airplane models.
First off, for the model, I would advise you to buy a Level 3 Airbus A320 for your nephew.
There are no Level 2 Commercial Airliners that I have found, and Level 2 would not require you to paint or sometimes even glue the model (alas, they are called "snap fit"). So Level 3 is basically your entry level kit for actually teaching you the basic skills needed to paint and glue the parts.
Here is a link for more info about this kit. The site is good for finding the history of models. https://www.scalemates.com/kits/revell-03942-airbus-a320-neo--1025102
Revell would be my first pick because the instructions are clear and the parts are well detailed for a beginner, but I will advise that there are no landing gears available.
My second pick would be Hasegawa's A320 ANA model. The manufacturer is from Japan, hence the airline ANA is from Japan.
Here is the link for more information: https://www.scalemates.com/kits/hasegawa-10828-airbus-a320neo-ana--1173295
I would like to tell you that this kit is old. Yes, it was made in 2018, but the molds that were used for the plastic sprues were from 2001, so fitment issues would be present.
But the thing that I like with this one is that it's simple. Not too many parts, so great for your nephew and for you.
However, the instructions are all in Japanese, but there is translation in English.
My last pick would be Zvezda's A320.
Link: https://www.scalemates.com/kits/zvezda-7037-airbus-a320neo--1327458
This is my last pick because it's the most difficult of them all, having lots of parts, and requires a little bit more experience. The instructions are in Russian, but there is English Translation.
Don't pick this kit until you both have experience.
As for tools, the only things you need are: side cutters, sandpaper, some painters tape, rubber bands, plastic cement, a box knife, a paintbrush, a good pair of tweezers, some paint cans, paint and thinner. Most of these you probably have in your house.
Side cutters - Honestly, you can just get away with some wire cutters laying around, or the nippers that you use for cuting your nails (Not nail clippers. Nippers.)
Sandpaper - regular sandpaper is fine, just use 600 grit for cleanup of rough edges and the gate.
Painters tape - When you have to paint, you have to mask off areas where you don't want paint. Also good if you want to hold your pieces in place as you're painting it, or when you're gluing it together.
Rubber bands - you guessed it, you can use this to clamp the model and hold it in place as it's drying from glue.
Plastic Cement - Testor's model glue is fine, but you could use some Tamiya's extra thin cement for smaller details. It works amazing. It's the one with the green bottle.
Box knife - For trimming the parts after you cut them off from the plastic tree.
Paintbrush - Just go to your local art store and buy a set of small paintbrushes of different sizes.
Tweezers - For grabbing tiny parts.
Paints
Paint cans - This is mostly just for the fuselage and the wings. Just buy some Tamiya paints and you'll be fine. Primer is also fine, Tamiya makes some great ones, or use rustoleum.
Paint - You should use some Tamiya Acrylic paint, and some Tamiya X-20A Acrylic Thinner. You have to make sure that it's acrylic and not enamel or lacquer.
If you are unsure of anything, whether that's model making techniques, how to do things, or just any questions, just use the forums and ask away. Most of these people are experienced modelers, and should have no problem helping you.
Last pieces of advice: Don't try and do a shopping spree and buy fancy tools just to make the model. Remember that this is just a model that you and your nephew will make for the first time, so I wouldn't expect both of you to make it perfectly.
Lastly, have fun! This will hepl your nephew be hooked in the hobby, and he'll start to explore it for himself.
I apologize for the large amount of text that I typed, I just want to help you start building with your nephew.