I haven't done all 4, but would LOVE to do all 4. I don't think it would be over kill at all. My kids (and I) had so much fun with CL it was amazing! But I haven't done all four, so I may be incorrect in my opinion after all. :)
I have done colonial life and have new world explorers. I wish I could just take the time off of other things and do these. I think all 4 together would be a GREAT American history for 1 year, just take 2 months do do each one. Believe me, there is enough information, suggested resources, etc. to take 2 months on each one.
It is for boys and girls a like. :) Yes, I woud think he would enjoy it. :)