You will want to visit the clan's website as much of this information probably will be found on it. Also -- there are designated boards for Clan interaction, no genealogy beyond about 1745 to current day, whereas the surname board is for genealogy of all eras and locations.
There is also a board for discussion of Coat of Arms & Heraldry:http://boards.rootsweb.com/topics.heraldry/mb.ashx
There are specific rules regarding Heraldry and Arms (shields). One must either earn them or inherit them, based on Heraldic Law. There is no "shield" or "family crest" for everyone.
I suspect that when you speak of "royalties" you are questioning if there are any heritable arms for your son. In order to discover this, you must do the genealogy of his Ancestors to find if any of them were Armigers and if the Arms descend to your son.
I find only 3 Arms granted to MacLellan in the General Armory. One has been dormant since 1832, one granted in Barclay, Scotland 1719 and a third granted in Edinburgh, Scotland 1685.
There may be others, but this is the book I have at hand for the moment. You'd have to do some work to discover if he may be entitled to register for any of them. You would have to petition the Lyon Court and pay the fees for EACH GENERATION to transfer the arms to your son.