If he is motivated to do something about it, he needs to know what his options are. However, as the old saying goes, you can lead a horse to water, but you can't make him drink. He has to want this for himself. He does need to know that there may be help out there for him. I can't answer if an undescended testicle could be a factor, but I doubt it.
Masturbation is an individual thing.
While it could help some people, it woudn't necessarily help others. MUSE can be helpful for some people. It can be painful for some people. My overall experience with it has been disappointing. We have seen that it is expensive, overall not very effective, and really uncomfortable for a small number of patients.