Hi Syracusa,
The Morgongava would certainly be on the firmer side of the range for most people although heavier body types would probably feel it was softer or in some cases even too soft than those who are lighter or children.
I would be tempted to give it a little time because a mattress that is too soft may not be the best choice for kids and he may find that it works well once he gets used to a different type of sleeping surface but if you decide that you want to add a little extra softness then post #2 here and the topper guidelines it links to should be helpful and it also links to the topper and component post here which includes some of the better sources for toppers that I’m aware of.
I’m not personally familiar with this mattress but I would guess that it may be even firmer than the mattress you purchased. A full size is also more costly than the Morgongava by over $350. Are you sure you mean My Organic Mattress and not My Green Mattress (which is one of the members of this site)?
Again I’m guessing you mean My Green Mattress? I don’t see a Pure Echo on the My Organic Mattress site.
Phoenix