D.R.
how about letting him plant and grow his own veggie garden. there is always something to be done, and it is really thrilling to harvest the fruits of your labors. awesome for outdoorsy kids. hatch some butterflies or praying mantises, also so cool. we are out there every day planting and weeding and watering and trimming etc, my son makes little bug houses, uses nets and magnifying glasses, etc "cares" for them (kills them, really,) but thats ok :) we recycle those containers that you buy lettuce or strawberries in for the bugs and for seedlings, though i think you are past the time for seedlings for most things, though sunflowers or corn or beans would still work... he recycled old juice boxes, drilled holes and planted things in them and sold them at our schools plant sale, we might do a lemonade stand version of a plant sale over the summer to deal with all the seedlings i have, ive been saving yogurt cups, he drills holes in the bottoms,,,, he loved it so much. ...
also, i dont know about your area, but by me the town/county parks offer lots of free or almost free summer programs for kids.
how about getting involved with some volunteer work? go in the direction of things he loves, maybe beach clean ups or something like that?