Whatever package you think you want, before you purchase take a free 30 day trial. It has all the features of the full edition.
Photoshop is an expensive and quite complex program, and not sure how you would do music editing.
Photoshop Elements has quite a lot of features , less daunting than Photoshop, and is less than $100.
Depending on the personality of the kid (is it a flash in the pan idea) I would also check out free programs on the web for what you want. Be advised that a one package fits all might not be the best as sometimes compromises are made to make it multi use. You might be better off getting a photo editing program, a video program and a music editing program, but look online for free stuff first and see how his interest level is..
I feel that you have gotten some great advice.
I would not pass on looking at the bundle of Adobe Premiere Elements (video-editing), and Photoshop Elements (Image-editing). The versions 11 have just been introduced, and are getting good reviews. I have not checked out the prices for the bundles on the Adobe Store, and certified resellers, like Amazon (direct from Amazon, and not a 3rd party), but the price for both programs, in the bundle, is usually around US $ 150, though they might have some "deals" with the Holidays approaching, and a new version release.
Both are very powerful programs, and offer much of that with user-friendly interfaces - less is done "by hand," as with Adobe Premiere CS 6 and Photoshop CS 6. Might be useful for a beginner.
It obviously largely depends at what level your son is currently at, as to what anybody can point you to with regards to appropriate software. As has already been said, the best idea would be to take advantage of the free trials for Photoshop, Elements etc etc.
Just to say, I have only recently moved up to Photoshop CS6. I began my photoshop 'journey' with Elements 8 and have used it for about the last four years. Elements seems to get much criticism for being thought of as just a toy, compared to the full version but I feel this is an exaggerated view. Sure, there are many things it cannot do (many ommissions only an intermediate or advanced level user might need) but that's why it is a fraction of the price of the full version. I think it makes complete sense to begin with a very reasonably priced, stripped down version of Photoshop, namely Elements, in order to develop some basic Photoshop skills, before taking the plunge into a more complete and sophisticated package such as Photoshop CS.
How would you feel if you went out and bought Photoshop CS straight away, then discovered your son rapidly lost interest? It makes much better sense to begin learning the basics of Photoshop, which he can do by using Elements, then move on to Photoshop CS when he is has reached a higher level.
I also have Premiere Elements for video editing and, like Elements, it can do a lot for a very reasonable price.
I can't advise about music software, this is not my area.