I understand what you're saying that others may not have seen.
An adult size is longer in the crotch seam along with being longer in the legs.
So altering a pair of adult pants means not only cutting the legs off but ALSO TAKING THE WAIST OFF AND CUTTING IT LOWER.
I have 2 little friends who struggle with this too. Luckily they are girls and can buy larger size dresses and cut them off. They can also buy knit pants and capris in the larger sizes and fold the waist over without pain.
Here's what I'd do.
In winter I'd buy a bunch of husky sweat pants that fit a little big around the waist, they do shrink some, and then I'd cut them off straight across on the bottom and hem them. He can wear them like jeans but have a drawstring for security and elastic for comfort, plus they stretch.
Then I'd buy a bunch of boys tops in the next larger size and fold the sleeves back or just buy short sleeves.
In summer I'd cut all those sweat pants off that are now probably too short. He can run around in them and play and wear them out the rest of the way.
I feel your pain. When a 6 year old little girl can't wear little girl clothes at all and has to go to the ladies department to get sweat pants and even then when she pulls them on her hiney crack shows...that's sad. But this little girl ate super "healthy". Her mom fixed salads and grilled their meats to reduce fat, they ate light things year round. This girl hit puberty and shot up and now looks like a normal kid.
The other little girl has growth spurts where she gets so large around she can't fit any of her clothes then one morning she wakes up and everything is too short and falling off her. She nearly has to have 2 wardrobes for each season.
Many stores like Sears and Penney and Target have plus size clothes for the kids. You didn't say if those clothes even come close but lets hope you can find some solutions in all these comments.