You mentioned in your post, “A lot of what he writes has an orchestral feeling.”
One question that comes to mind is: Does your son have access to an orchestra?
If the answer is yes, then Dorico would make a good choice because it produces the best sight-reading ready engraving you can get today.
If the answer is that much of what your son wishes to do will depend on the scoring program’s playback for orchestral work, then I (reluctantly and not desiring to be “provocative”) cannot recommend Dorico at this time or even after the next update based on what is known today. If he needs a reasonable simulation of orchestral playback from score, then I would recommend (despite the remarks concerning competing products made here) a copy of Sibelius and Noteperformer (a separate product offered here: http://www.noteperformer.com). The strength of the recommendation is made solely on the playback issue. For engraving, Dorico will likely beat Sibelius every time (after the upcoming update which will complete support for unpitched percussion and a few other significant details in orchestral scores.)
There are several YouTube videos available from Wallender instruments with classical scores for your listening evaluation. You can download Dorico’s trial with Halion SE instruments and judge for yourself.
Later, if Noteperformer should come to support Dorico (the author is apparently waiting for some maturing of Dorico’s playback and interpretation functions), your son can switch to Dorico at a discounted price since Steinberg has announced that it is continuing it’s discounts for Sibelius crossgrades “indefinitely.” This (in my mind) lowers the barrier of chainging over to Dorico when it is more mature.
I would also talk (if possible) to one of your son’s instructors and find out what they use.
I own (and use) all three of the product you mentioned in your original post and compose primarily music for full orchestra. From the point of view of what a beginner in orchestration and arrangement needs, I feel they can be well served today by either Dorico or SIbelius (both were created by the team currently working on Dorico.) Finale would potentially be more frustrating, but I do know several users who still produce material in it every week.
I think if you asked me the same question two years from now, I would speculate that the answer will be Dorico; however I do not believe it is sufficiently mature today and will potentially create unnecessary limits on a beginning composer.
Regardless of your choice, both firms can offer you educational discounts for your son and you should take full advantage of these offers. I can also echo Daniel’s recommendation of Sweetwater in the US. They have a very knowledgeable staff and can help you navigate to the best deal for quite a variety of music products.