"My son claims that it does, but I find this hard to believe. He is 25 years old so can make his own decisions, but is there any validity to this claim?"
Yes cannabis can improve concentration and focus and is used by many individuals as an alternative to ADD/ADHD medication. I have spoken with many patients who would verify your son's claim. Most often they report that it helps them stay "in the zone" and avoid distraction, enabling them to complete their desired task, instead of being sidetracked by negative thinking, doubts and other stimuli. However, this effect is strain and dose specific. Some people report that certain strains hinder their ability to focus, cause scattered thinking and an overall decrease in productivity. I would reccomend microdosing, which is a very small dose of cannabis (on the order of a few milligrams, or half a puff, or a very small portion of an edible). Cannabis seems to affect everyone in a slightly different fashion, depending on strain, dose, set and setting and delivery method. Cannabis raises consiousness, and for some this makes everyday reality a bit more interesting, and can make work and focusing on a task or project more enjoyable and less of a drudgery.