Hello and welcome to the community!
Since your son has a 'long' sleeping bag, it is likely safe to assume he is also tall enough to need a 'long' sleeping pad. Some people opt for a shorter sleeping pad to save on weight and either let their feet hang off the end of the pad or sometimes place their backpack under the end of the pad to help keep their feet off the ground. The width of the sleeping pad, whether you go with a 'wide' or a 'regular' will fit his sleeping bag just fine. However, there are several things to bear in mind when considering a 'wide' pad:
- Wide sleeping pads are typically five inches wider, at 25", than a regular sleeping pad, which measure at 20". If your son has broad shoulders and sleeps on his back or stomach, or rolls around at night, those extra five inches can be pretty helpful in staying on the sleeping pad.
- The number of people who fit into tents, particularly backpacking tents, are often determined by how many 20" sleeping pads can fit inside the tent with a couple of inches of room on either side. As an example, the Big Agnes Tiger Wall UL 2 tent is considered a two person tent and is 52" wide at its widest. As such, if two people were using the tent and had 'wide' 25" sleeping pads, there would only be two inches of 'extra space', amounting to 2/3" between the pads and on each side. That would be a pretty cramped tent, not to mention that the tent tapers down to only 42" at the foot end! This is why many sleeping pads are also 'mummy' shaped with a taper at the bottom, to accommodate a similar shape in the tent.
- With the additional width of the sleeping pad you are adding extra weight and bulk to what you are carrying. Sometimes that can be negligible, at seven ounces difference between the long-wide and the regular in this case, but if you are an 'ounce counter' that's almost a half a pound of weight you are adding just in your sleeping pad.
As we look at the options available, it looks like all the sleeping pads that come in 'long' are also 'wide'. Personally, I have used a 'wide' sleeping pad for years and appreciate the comfort I get sleeping enough that it warrants the extra weight and bulk. For reference, I have broad shoulders at around 20" (shoulder joint to shoulder joint) and stand at about 6'1" tall. I am a side sleeper and tend to roll over multiple times each night. I also use tents wide enough to have plenty of room for a wide sleeping pad.
It sounds like a long wide pad would work for your son, particularly if he is tall enough to need those extra inches. As such, the NEMO Flyer Sleeping pad in long wide would be a great option. If he is very focused on the weight of his pack, isn't concerned about his sleeping bag and feet hanging off the end, and is using a smaller, ultralight tent that isn't very wide, you may want to consider a regular width sleeping pad.
Hopefully this helps, thanks!