Advise on wheelie bike for 12yo son

Non-riding Dad here. My son (age 12, 4'8", 72 lbs) has a 24 inch Trek mountain bike and can pop his front tire a bit, but can't yet do wheelies, which he's trying hard to do. We are looking at something lighter, but aren't sure what to get, preferably in the $300 or less range.

We are looking at https://www.citygrounds.com/products/gt-bikes?variant=41098564829295, but would love suggestions. We tried a 20" Mongoose at Dicks Sporting Goods and he likes it, but neither of us are sure what's best. We are also at odds on 20" vs 24" tires.

Also: How much is a skill issue vs having the wrong bike for the job? He's pretty light weight, so I'd wager he'd need a lighter bike to control the balance better. Will a different bike make a dramatic difference, or does he just need to practice more?

Thanks for any advice you can give!