Oversized tyres and teaching my son to measure!
Regarding diameter: You can't put 26&quot; tires on a 27.5&quot; rim, which it sounds like your son wants to do. I can guarantee with absolute certainty that the new tires will not work because they're the wrong diameter.



Regarding width: We don't know the width of the rims on your son's bike.
Also, rims can handle tires across a range of widths. It is possible that the 2.1&quot; tires on the rims now are at the outer limit of the rims' range, but we have no way of knowing. I'd look at the rim and look up the manufacturer's guidelines online.
Regarding removing the existing tires: do you have tire levers? There are numerous videos showing how to remove a tire. Here's one.