Oh dear, good luck starting down that road! They are very addictive, you know. smile DS was 5 or so when he started on calculators, and now he has a dozen different ones and still craves more. I think the one that he has that you might like is the Sharp WriteView; it's one of his favorites. There are multiple models of it, but we got one for about $20.

Don't forget to get him a slide rule to go with them!