Hello Warren,
The best thing to do is to put the styles on an USB stick and try to play them trough the DGX 660. Some will play fine, some will sound strange and will need some tweaking if you want to use them and some will not work at all, for instance if they are too big or contain mega voices etc.
I suggest that you write down which styles are working, which don't and which you are wiling to tweak. It is wise to delete the styles you don't want to keep because the DGX 650 can only accomodate 100 style files on one USB stick (you can store more of course but the DGX will only "see" the first 100 in alphabetical order).
The DGX will read files into folders so this can help you sort the styles by category.
If you like a particular PSR S970 style and you want to keep the 4 variations, you will have to use Style Remixer to "divide" your style into two new styles. For instance, if you have a style labelled "Rock" from the PSR S970, you can put intro 1 and ending 1 together with variations 1 and 2 in a new "Rock1" style, and put intro 2 and ending 2 as well as variations 3 and 4 in a new "Rock2" style. This way, you can achieve some variety.
And: just dig into the "Styles" section of the Website to find new styles for your son's DGX 660. Usually "older" keyboards styles will work, but sometimes there will be tempo or volume issues, just give it a try, for instance with styles for the PSR 740.
Hope this helps,
Regards,
Vinciane.