i don't have a suggestion for you as to a computer spanish language disc but i have long liked the idea of teaching kids 2 languages. i read that its easier to teach younger children a new language than it is to teach older ones.
Umm, hate to tell you this, but it's almost impossible to pick up a second language on the computer. Your son needs to be in a situation where he can HEAR the language being spoken (preferably by a native speaker) and where he can RESPOND verbally. I would start looking for some sort of introductory Spanish classes for elementary aged children.
